Revenue from Contract with Customer [Abstract] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Project [Axis] Project [Axis] Project [Domain] Project [Domain] Teva Teva Pharmaceuticals Collaboration Agreement [Member] Teva Pharmaceuticals Collaboration Agreement [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Unsatisfied obligation Unsatisfied Obligation Unsatisfied Obligation Supplemental Cash Flow Information [Abstract] Schedule of Cash and Cash Equivalents Schedule of Cash and Cash Equivalents [Table Text Block] Restrictions on Cash and Cash Equivalents Restrictions on Cash and Cash Equivalents [Table Text Block] Investments, Debt and Equity Securities [Abstract] Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-sale [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] Corporate bonds Corporate Bond Securities [Member] Certificates of deposit Certificates of Deposit [Member] U.S. government and government agency obligations US Treasury and Government [Member] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Fair Value - Less than 12 Months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months Unrealized Loss - Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Fair Value - 12 Months or Greater Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer Unrealized Loss - 12 Months or Greater Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Fair Value - Total Debt Securities, Available-for-sale, Unrealized Loss Position Unrealized Loss - Total Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss Realized gain (loss) on sales of marketable securities Marketable Securities, Realized Gain (Loss) Revenues [Abstract] Product Sales Product Sales and Concentration Risk [Text Block] Product Sales and Concentration Risk Disclosure [Text Block] Earnings Per Share [Abstract] Schedule of Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Table] Schedule of Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Table] Award Type [Axis] Award Type [Axis] Award Type [Domain] Award Type [Domain] Stock options Share-based Payment Arrangement, Option [Member] Restricted stock Restricted Stock [Member] Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items] Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items] Net income - basic and diluted Net Income (Loss) Attributable to Parent Weighted average shares outstanding - basic (in shares) Weighted Average Number of Shares Outstanding, Basic Effect of dilutive securities (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Weighted average shares - diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Net income per share - basic (in dollars per share) Earnings Per Share, Basic Net income per share - diluted (in dollars per share) Earnings Per Share, Diluted Accounts receivable - other Accounts and Financing Receivable, after Allowance for Credit Loss Other liabilities Other Liabilities Schedule of collaboration revenue Collaborative Arrangement and Arrangement Other than Collaborative [Table Text Block] Summary of contract balances Contract With Customer and Collaborator Asset And Liability [Table Text Block] Contract With Customer and Collaborator Asset And Liability [Table Text Block] Maturities within one year Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Maturities after one year through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Total Debt Securities, Available-for-sale Organization, Consolidation and Presentation of Financial Statements [Abstract] Schedule of Error Corrections and Prior Period Adjustment Restatement [Table] Schedule of Error Corrections and Prior Period Adjustment Restatement [Table] Restatement [Axis] Restatement [Axis] Restatement [Domain] Restatement [Domain] As Previously Reported Previously Reported [Member] Adjustments Restatement Adjustment [Member] Error Corrections and Prior Period Adjustments Restatement [Line Items] Error Corrections and Prior Period Adjustments Restatement [Line Items] Increase in deferred revenue Increase (Decrease) in Contract with Customer, Liability (Decrease) increase in accounts payable, accrued expenses, and other liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Schedule of product sales Disaggregation of Revenue [Table Text Block] Schedules of Concentration of Risk, by Risk Factor Schedules of Concentration of Risk, by Risk Factor [Table Text Block] Fair Value Disclosures [Abstract] Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Corporate bonds U.S. government and government agency obligations Sovereign bonds Sovereign Debt Securities [Member] Commercial paper Commercial Paper [Member] Certificates of deposit Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Estimate of Fair Value Measurement Estimate of Fair Value Measurement [Member] Classification of Investment [Axis] Classification of Investment [Axis] This element represents the general categories of investments, which may be classified as Unrestricted or Restricted. Classification of Investment [Domain] Classification of Investment [Domain] Provides general categories of investments for which information is required or determined to be disclosed. Unrestricted Unrestricted [Member] Investments whose use is not restricted in whole or in part, by contractual agreements or regulatory requirements. Restricted Restricted [Member] Restricted [Member] Measurement Frequency [Axis] Measurement Frequency [Axis] Measurement Frequency [Domain] Measurement Frequency [Domain] Measured on a recurring basis Fair Value, Recurring [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Available-for-sale debt securities: Equity securities Equity Securities, FV-NI Total marketable securities Marketable Securities Collaboration Agreements Collaborative Arrangement Disclosure [Text Block] Inventory Disclosure [Abstract] Schedule of Inventory, Current Schedule of Inventory, Current [Table Text Block] Equity [Abstract] Stockholders' Equity Stockholders' Equity Note Disclosure [Text Block] Inventories Inventory Disclosure [Text Block] Basic and Diluted Net Income (Loss) Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Antidilutive Securities Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Sanofi Collaboration Agreement, Antibody Sanofi Collaboration Agreement, Antibody [Member] Sanofi Collaboration Agreement, Antibody [Member] Accounts receivable, net Accounts Receivable, Related Parties, Current Deferred revenue Contract with Customer, Liability Statement of Comprehensive Income [Abstract] Statement [Table] Statement [Table] Sanofi Sanofi Collaboration Agreement [Member] Sanofi Collaboration Agreement [Member] Bayer Bayer Collaboration Agreement [Member] Bayer Collaboration Agreement [Member] Other Other Revenue [Member] Other Revenue [Member] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Product Product [Member] Product and service, other Product and Service, Other [Member] Collaboration and contract manufacturing Collaboration and Contract Manufacturing [Member] Collaboration and Contract Manufacturing [Member] Statement [Line Items] Statement [Line Items] Statements of Operations Income Statement [Abstract] Revenues: Revenues Revenue from Contract with Customer, Excluding Assessed Tax Expenses: Costs and Expenses [Abstract] Research and development Research and Development Expense Selling, general, and administrative Selling, General and Administrative Expense Cost of goods sold Cost of Goods and Services Sold Other operating (income) expense, net Other Operating Income (Expense), Net Total expenses Costs and Expenses Income from operations Operating Income (Loss) Other income (expense): Nonoperating Income (Expense) [Abstract] Other (expense) income, net Other Nonoperating Income (Expense) Interest expense Interest Expense Total other income (expense) Nonoperating Income (Expense) Income before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax expense Income Tax Expense (Benefit) Net income Weighted average shares - diluted (in shares) Statements of Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other comprehensive income (loss), net of tax: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Unrealized (loss) gain on debt securities Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Unrealized loss on cash flow hedges Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Statement of Financial Position [Abstract] Class of Stock [Axis] Class of Stock [Domain] Class A Stock Common Class A [Member] Common Stock Common Stock [Member] LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities and Equity [Abstract] Stockholders' Equity: Stockholders' Equity Attributable to Parent [Abstract] Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Preferred stock, issued (in shares) Preferred Stock, Shares Issued Preferred stock, outstanding (in shares) Preferred Stock, Shares Outstanding Common Stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common Stock, shares authorized (in shares) Common Stock, Shares Authorized Common Stock, shares issued (in shares) Common Stock, Shares, Issued Common Stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Treasury stock, outstanding (in shares) Treasury Stock, Shares Sanofi Collaboration Agreement, Immuno-oncology Sanofi Collaboration Agreement, Immuno-oncology [Member] Sanofi Collaboration Agreement, Immuno-oncology [Member] Commitments and Contingencies Disclosure [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] '165 Patent '165 Patent [Member] '165 Patent [Member] '741 Patent '741 Patent [Member] '741 Patent [Member] '487 Patent '487 Patent [Member] '487 Patent [Member] Litigation Case [Axis] Litigation Case [Axis] Litigation Case [Domain] Litigation Case [Domain] Amgen v.s. Regeneron Amgen v.s. Regeneron [Member] Amgen v.s. Regeneron [Member] Litigation Status [Axis] Litigation Status [Axis] Litigation Status [Domain] Litigation Status [Domain] Pending Litigation Pending Litigation [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Loss contingency accrual Loss Contingency Accrual Pending claims Loss Contingency, Pending Claims, Number Invalid claims Loss Contingency, Claims Invalid, Number Loss Contingency, Claims Invalid, Number Loss contingency, damages sought, per unit produced Loss Contingency, Damages Sought, per unit produced. Loss Contingency, Damages Sought, per unit produced. Loss contingency, damages sought Loss Contingency, Damages Sought, Value Number of IPR filed Gain Contingency, Inter Partes Review Filed, Number Gain Contingency, Inter Partes Review Filed, Number Number of claims invalidated Gain Contingency, Claims Invalidated, Number Gain Contingency, Claims Invalidated, Number Sanofi Collaboration Agreement, Immuno-oncology Maximum shares the collaborator could sell (in shares) Maximum shares the collaborator could sell Maximum shares the collaborator could sell through September 30, 2020. Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Statement of Cash Flows Cash Flow, Supplemental Disclosures [Text Block] Corporate bonds U.S. government and government agency obligations Sovereign bonds Commercial paper Amortized Cost Basis Debt Securities, Available-for-sale, Amortized Cost Unrealized Gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Unrealized Losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Fair Value Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Weighted average number of shares (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Concentration Risk [Table] Concentration Risk [Table] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Gross Sales Revenue Gross Sales Revenue [Member] Gross Sales Revenue [Member] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Customer concentration risk Customer Concentration Risk [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Besse Medical, a subsidiary of AmerisourceBergen Corporation Besse Medical [Member] Besse Medical [Member] McKesson Corporation McKesson Corporation [Member] McKesson Corporation [Member] Concentration Risk [Line Items] Concentration Risk [Line Items] Concentration risk, percentage Concentration Risk, Percentage Reimbursement of research and development expenses Reimbursement Of Research And Development Expenses [Member] Reimbursement Of Research And Development Expenses [Member] Amounts recognized in connection with up-front and development milestone payments received Amounts Recognized In Connection With Up-Front Payments [Member] Amounts Recognized In Connection With Up-Front Payments [Member] Reduction of Research and development expense Reimbursement Of Research And Development Expense Reimbursement Of Research And Development Expense Other operating income Other Operating Income Basis of Presentation Basis of Accounting, Policy [Policy Text Block] Use of Estimates Use of Estimates, Policy [Policy Text Block] Recently Issued Accounting Standards New Accounting Pronouncements, Policy [Policy Text Block] Collaboration Agreements Revenue from Contract with Customer [Policy Text Block] Legal Matters Legal Costs, Policy [Policy Text Block] Net unrealized gain (loss) on securities Equity Securities, FV-NI, Unrealized Gain (Loss) Securities owned not readily marketable Equity Securities without Readily Determinable Fair Value, Amount Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Share Repurchase Program [Domain] Share Repurchase Program [Domain] Share Repurchase Program Share Repurchase Program [Member] Share Repurchase Program [Member] Libtayo development Libtayo Development [Member] Libtayo Development [Member] Dupilumab/REGN3500 Eligible Investments Dupilumab/REGN3500 Eligible Investments [Member] Dupilumab/REGN3500 Eligible Investments [Member] Class of Stock [Line Items] Class of Stock [Line Items] Share repurchase program, authorized amount Stock Repurchase Program, Authorized Amount Treasury stock, shares acquired (in shares) Treasury Stock, Shares, Acquired Cost of treasury stock shares received Treasury Stock, Value, Acquired, Cost Method Remaining authorized repurchase amount Stock Repurchase Program, Remaining Authorized Repurchase Amount Interim Financial Statements Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Legal Matters Legal Matters and Contingencies [Text Block] Accrued capital expenditures Accrued Capital Expenditures Carrying value as of the balance sheet date of obligations incurred and payable for purchases of fixed assets. Included in accounts payable and accrued expenses. Fair Value Measurements Fair Value Disclosures [Text Block] Net Income Per Share Earnings Per Share [Text Block] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Other Other Collaboration Agreements [Member] Other Collaboration Agreements [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Accrued expenses and other current liabilities Accrued Liabilities and Other Liabilities Deferred revenue, current Contract with Customer, Liability, Current Other liabilities - Sanofi Other Liabilities, Current Deferred revenue, noncurrent Contract with Customer, Liability, Noncurrent Other noncurrent liabilities Other Liabilities, Noncurrent Available-for-sale Debt Securities Debt Securities, Available-for-sale [Table Text Block] Marketable Securities, Based on Contractual Maturity Dates Investments Classified by Contractual Maturity Date [Table Text Block] Fair Value and Unrealized Losses of Marketable Securities Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value [Table Text Block] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Revenue based on percentage of annual sales in Japan Revenue based on percentage of annual sales in Japan The percentage on annual sales in Japan that the Company is entitled to receive, subject to certain potential adjustments. Statement of Cash Flows [Abstract] Cash flows from operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Non-cash compensation expense Share-based Payment Arrangement, Noncash Expense Other non-cash items, net Other Noncash Income (Expense) Deferred taxes Increase (Decrease) in Deferred Income Taxes Changes in assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Increase in Sanofi, trade, and other accounts receivable Increase (Decrease) in Accounts Receivable Increase in inventories Increase (Decrease) in Inventories Decrease in prepaid expenses and other assets Increase (Decrease) in Prepaid Expense and Other Assets Total adjustments Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Cash flows from investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Purchases of marketable and other securities Payments to Acquire Marketable Securities Sales or maturities of marketable securities Proceeds from Sale and Maturity of Marketable Securities Capital expenditures Payments to Acquire Productive Assets Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Cash flows from financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from issuance of Common Stock Proceeds from Issuance of Common Stock Payments in connection with Common Stock tendered for employee tax obligations Payment, Tax Withholding, Share-based Payment Arrangement Repurchases of Common Stock Payments for Repurchase of Common Stock Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Net increase in cash, cash equivalents, and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash, cash equivalents, and restricted cash at beginning of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, cash equivalents, and restricted cash at end of period Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Outside United States Non-US [Member] Regeneron's net profit in connection with commercialization of EYLEA outside the United States Net Profit In Connection With Commercialization [Member] Net Profit In Connection With Commercialization [Member] Reimbursement for manufacturing of commercial supplies Reimbursement For Manufacturing Of Commercial Supplies [Member] Reimbursement For Manufacturing Of Commercial Supplies [Member] Reimbursement of development expenses Regeneron's obligation for its share of Bayer research and development expenses Regeneron's Share Of Research And Development Expenses [Member] Regeneron's Share Of Research And Development Expenses [Member] Reimbursement of other expenses Other [Member] Other [Member] Reduction of Research and development expense Research and development expense Cost of collaboration and contract manufacturing Reimbursement Of Other Expenses Reimbursement Of Other Expenses Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Restricted cash included in Other noncurrent assets Restricted Cash Total cash, cash equivalents, and restricted cash shown in the Condensed Consolidated Statement of Cash Flows Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Sanofi Sanofi [Member] Sanofi [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Praluent Agreement Praluent Cross License And Commercialization Agreement [Member] Praluent Cross License And Commercialization Agreement [Member] Percentage of Trial Costs borne by collaborating party Percentage of Trial Costs borne by collaborating party The percentage of subsequent Shared Phase 3 Trial costs borne by the collaborating party upon receipt of the first positive Phase 3 trial results for a co-developed drug candidate. Percentage of Trial Costs borne by entity Percentage of Trial Costs borne by entity The percentage of subsequent Shared Phase 3 Trial costs borne by the Company upon receipt of the first positive Phase 3 trial results for a co-developed drug candidate. Maximum amount of sales milestone payments if total sales achieve specific levels Maximum amount of sales milestone payments if total sales achieve specific levels The maximum amount of sales milestone payments the Company is eligible to receive when and if total annual sales achieve certain specified levels. Levels of twelve month sales at which sales milestone payments would be received Levels of twelve month sales at which sales milestone payments would be received Aggregate amount of twelve month sales upon which sales milestone would be received. Period for achieving sales target for milestone payment, rolling basis Period for Achieving Sales Target for Milestone Payment, Rolling Basis Period for Achieving Sales Target for Milestone Payment, Rolling Basis Royalty percentage to be received on net product sales outside of the United States Royalty Percentage To Be Received On Net Product Sales Outside Of United States Royalty Percentage To Be Received On Net Product Sales Outside Of United States Percentage of royalty payment that can be used to offset litigation proceedings Percentage Of Royalty Payment That Can Be Offset Percentage Of Royalty Payment That Can Be Offset Percentage of damages company is responsible for Loss Contingency, Percentage Of Damages Responsible For Loss Contingency, Percentage Of Damages Responsible For Schedule of Reclassifications Schedule of Error Corrections and Prior Period Adjustments [Table Text Block] Cover [Abstract] Entities [Table] Entity Information [Line Items] Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Marketable Securities Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Effective income tax rate Effective Income Tax Rate Reconciliation, Percent Total operating expenses Common Stock ASSETS Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Marketable securities Marketable Securities, Current Accounts receivable - trade, net Accounts Receivable, after Allowance for Credit Loss, Current Accounts receivable - Sanofi Inventories Inventory, Net Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Total current assets Assets, Current Marketable securities Marketable Securities, Noncurrent Property, plant, and equipment, net Property, Plant and Equipment, Net Deferred tax assets Deferred Income Tax Assets, Net Other noncurrent assets Other Assets, Noncurrent Total assets Assets LIABILITIES AND STOCKHOLDERS' EQUITY Current liabilities: Liabilities, Current [Abstract] Accounts payable Accounts Payable Total current liabilities Liabilities, Current Finance lease liabilities Finance Lease, Liability, Noncurrent Total liabilities Liabilities Stockholders' equity: Preferred Stock, $.01 par value; 30,000,000 shares authorized; issued and outstanding - none Preferred Stock, Value, Issued Common stock Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive (loss) income Accumulated Other Comprehensive Income (Loss), Net of Tax Treasury Stock, at cost; 5,683,562 shares in 2020 and 4,860,123 shares in 2019 Treasury Stock, Value Total stockholders' equity Stockholders' Equity Attributable to Parent Total liabilities and stockholders' equity Liabilities and Equity Statement of Stockholders' Equity [Abstract] Class of Stock [Axis] Class of Stock [Domain] Common Stock Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Additional Paid-in Capital Additional Paid-in Capital [Member] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Treasury Stock Treasury Stock [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning Balance (in shares) Shares, Outstanding Beginning Balance Issuance of Common Stock for equity awards granted under long-term incentive plans (in shares) Shares Issued, Shares, Share-based Payment Arrangement, after Forfeiture Issuance of Common Stock for equity awards granted under long-term incentive plans Shares Issued, Value, Share-based Payment Arrangement, after Forfeiture Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations (in shares) Share-based Payment Arrangement, Shares Withheld for Tax Withholding Obligation Common Stock tendered upon exercise of stock options and vesting of restricted stock for employee tax obligations Share-based Payment Arrangement, Decrease for Tax Withholding Obligation Issuance/distribution of Common Stock for 401(k) Savings Plan Stock Issued/Distributed During Period, Value, Employee Benefit Plan Stock Issued/Distributed During Period, Value, Employee Benefit Plan Issuance/distribution of Common Stock in connection with 401(k) Savings Plan (in shares) Stock Issued/Distributed During Period, Shares, Employee Benefit Plan Stock Issued/Distributed During Period, Shares, Employee Benefit Plan Repurchases of Common Stock (in shares) Repurchases of Common Stock Stock-based compensation charges APIC, Share-based Payment Arrangement, Increase for Cost Recognition Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Adjustment upon adoption of new accounting standard Cumulative Effect of New Accounting Principle in Period of Adoption Ending Balance (in shares) Ending Balance UNITED STATES UNITED STATES EYLEA EYLEA Net Product Sales [Member] Revenue during the period derived from a specified product line (EYLEA), after deducting applicable provisions for discounts, returns, rebates, chargebacks, distribution fees, and other sales-related deductions. Libtayo LIBTAYO Net Product Sales [Member] LIBTAYO Net Product Sales [Member] ARCALYST Arcalyst Net Product Sales [Member] Revenue during the period derived from a specified product line (ARCALYST), after deducting applicable provisions for discounts, returns, rebates, chargebacks, distribution fees, and other sales-related deductions. Schedule of assets measured at fair value on a recurring basis Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Table Text Block] Regeneron's share of profits (losses) in connection with commercialization of antibodies Share Of Profit (Losses) In Connection With Commercialization [Member] Share Of Profit (Losses) In Connection With Commercialization [Member] Regeneron's obligation for its share of Sanofi research and development expenses Reimbursement of commercialization-related expenses Reimbursement Of Commercialization-Related Expenses [Member] Reimbursement Of Commercialization-Related Expenses [Member] Regeneron's obligation for Sanofi's share of Libtayo U.S. gross profits Sanofi's Share of Libtayo U.S. Gross Profits [Member] Sanofi's Share of Libtayo U.S. Gross Profits [Member] Amounts recognized in connection with up-front payments received Sanofi Collaboration Agreement, Immuno-oncology Reduction of Selling, general, and administrative expense Reimbursement Of Selling, General And Administrative Expense Reimbursement Of Selling, General And Administrative Expense Cost of goods sold Raw materials Inventory, Raw Materials, Net of Reserves Work-in-process Inventory, Work in Process, Net of Reserves Finished goods Inventory, Finished Goods, Net of Reserves Deferred costs Other Inventory, Net of Reserves Total Inventories