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Document Period End Date Entity Emerging Growth Company us-gaap_DebtInstrumentFaceAmount Debt Instrument, Face Amount Document Type Entity Small Business bcrx_AverageMaturityForPortfolioInvestments Average Maturity for Portfolio Investments Average maturity for portfolio investments during the period. Document Information [Line Items] Document Information [Table] Entity Filer Category Debt Instrument [Axis] Entity Current Reporting Status Debt Instrument, Name [Domain] London Interbank Offered Rate (LIBOR) [Member] US Department of Health and Human Services [Member] United States department of health and human services. 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Additional Development Options [Member] The additional development options that can be exercised by the Government. ASPRBARDA Contract [Member] The potential value of the ASPR/BARDA Contract. Entity Common Stock, Shares Outstanding (in shares) bcrx_BusinessCombinationAgreementTerminatedExpectedTransactionrelatedExpenses Business Combination, Agreement Terminated, Expected Transaction-related Expenses The expected transaction-related expenses that are expected to reimburse to the acquiree due to the merge agreement terminated. Green Cross Corporation [Member] Represents Green Cross Corporation. bcrx_MilestonePaymentMinimum Milestone Payment Minimum Represents milestone payment minimum. bcrx_TransactionCosts Transaction Costs Represents transaction costs associated with royalty monetization. bcrx_InterestReserve Interest Reserve Represents interest reserve established as part of the royalty monetization. bcrx_MilestonePaymentMaximum Milestone Payment Maximum Represents milestone payment maximum. bcrx_RequiredForeignCurrencyHedgePerDollar Required Foreign Currency Hedge Per Dollar The Company is required to maintain a foreign currency hedge per dollar under the agreements governing the PhaRMA Notes. Sales Revenue, Product Line [Member] Royalty Monetization [Member] Represents the royalty monetization transaction. National Institute of Allergy and Infectious Diseases [Member] Represents National Institute of Allergy and Infectious Diseases. us-gaap_IncreaseDecreaseInInventories Inventory Trading Symbol Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans Stock Issued During Period, Shares, Employee Stock Purchase Plans us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd Line of Credit Facility, Interest Rate at Period End us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised Options outstanding, stock option awards exercised (in shares) us-gaap_TableTextBlock Notes Tables Currency Hedge Agreement [Member] Represents the Currency Hedge Agreement to hedge certain risks associated with changes in the value of the Japanese yen relative to the U.S. dollar. bcrx_MaximumAggregateOfferingPrice Maximum Aggregate Offering Price Represents the maximum aggregate offering price of common stock, preferred stock, depository shares, stock purchase contracts, warrants and units. bcrx_AnnualLicenseFeeMinimum Annual License Fee Minimum Represent minimum annual license fee. bcrx_AnnualLicenseFeeMaximum Annual License Fee Maximum Represents annual maximum license fee. us-gaap_LossOnSaleOfInvestments Loss on Sale of Investments General and administrative bcrx_NumberOfStockbasedCompensationPlans Number of Stock-based Compensation Plans Represents number of stock based compensation plan. us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Options outstanding, stock option awards granted (in shares) Line of Credit Facility, Lender [Domain] Arrangements and Non-arrangement Transactions [Domain] bcrx_SharebasedCompensationArrangementBySharbasedPaymentAwardAwardsUnvestedPercentage Share-based Compensation Arrangement by Shar-based Payment Award, Awards Unvested, Percentage Represents the percentage of performance awards that remain unvested. bcrx_AdvanceNoticePeriodForTerminationOfAgreement Advance Notice Period for Termination of Agreement Represents advance notice period for termination of agreement. us-gaap_StockIssuedDuringPeriodSharesNewIssues Stock Issued During Period, Shares, New Issues Lender Name [Axis] bcrx_PercentageOfSalaryToPurchaseCommonStockMaximum Percentage of Salary to Purchase Common Stock, Maximum Represents percentage of salary to purchase common stock maximum. bcrx_PercentageOfCommonStockSharesBeginning Percentage of Common Stock Shares, Beginning Represents percentage of common stock shares beginning. us-gaap_LiabilitiesAndStockholdersEquity Total liabilities and stockholders’ equity bcrx_PercentageOfCommonStockSharesEnding Percentage of Common Stock Shares, Ending Represents percentage of common stock shares ending. bcrx_ShareBasedCompensationArrangementBySharbasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount Shar-based Compensation Arrangement by Shar-based Payment Award, Maximum Number of Shares Per Employee, Amount Represents share-based compensation arrangement by share-based payment award maximum value per employee amount. 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Research and Development Expense, Policy [Policy Text Block] RAPIVAB [Member] Represents RAPIVAB product. us-gaap_AvailableForSaleSecuritiesDebtSecuritiesNoncurrent Investments Depreciation and amortization us-gaap_SharesIssuedPricePerShare Shares Issued, Price Per Share us-gaap_AssetsCurrent Total current assets Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Stockholders' Equity Note Disclosure [Text Block] bcrx_MaturityPeriodOfHighQualityMarketablesecurities Maturity Period of High Quality Marketable Securities Represents maturity period of high quality marketable securities. Centers for Disease Control and Prevention [Member] The Centers for Disease Control and Prevention (CDC) is a federal agency that conducts and supports health promotion, prevention and preparedness activities in the United States, with the goal of improving overall public health. Accrued Interest Represents the amount of accrued interest in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Common stock, $0.01 par value: shares authorized — 200,000; shares issued and outstanding — 109,625 in 2018 and 98,411 in 2017 Adjustments to reconcile net loss to net cash used in operating activities: Common stock, shares authorized (in shares) Common stock, shares issued (in shares) Certificates of Deposit [Member] Common stock, par value (in dollars per share) Deferred collaboration expense Range [Domain] Maximum [Member] Available For Sale Securities Debt Maturities Fair Value [Table Text Block] Tabular disclosure of available for sale securities debt maturities fair value. Minimum [Member] Maturing after one year through two years Represents available for sale securities debt maturities after one through two years fair value. Product and Service [Axis] Schedule of Receivables from Collaborations [Table Text Block] Tabular disclosure representing receivables from collaborations. Product and Service [Domain] Range [Axis] Shionogi and Co. Ltd [Member] Shionogi & Co. Ltd. Preferred stock, $0.001 par value; shares authorized — 5,000; no shares issued and outstanding CSL Limited [Member Represents CSL Limited which is a company organized under the laws of Australia. Preferred stock, shares issued (in shares) Intangible Assets, Finite-Lived, Policy [Policy Text Block] Preferred stock, shares authorized (in shares) Inventory Preferred stock, par value (in dollars per share) bcrx_RoyaltyTerm Royalty Term Period of time between the date of the Agreement and its expiration. Revenues Fair Value Hierarchy and NAV [Domain] Currency [Axis] Customer [Axis] Customer [Domain] bcrx_ContractTerm Contract Term Period of time between the contract date and its expiration. 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Interest Expense and Deferred Financing Costs [Policy Text Block] Represents interest expense and deferred financing costs. Currency Hedge Agreement [Policy Text Block] Represents currency hedge agreement policy. us-gaap_OperatingIncomeLoss Loss from operations us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash used in operating activities Concentration of Market Risk [Policy Text Block] Represents concentration of market risk policy. Prepaid expenses and other current assets us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash provided by (used in) investing activities Cost of products sold Counterparty Name [Axis] Counterparty Name [Domain] Accounting Standards Update 2014-09 [Member] Investment, Policy [Policy Text Block] Type of Adoption [Domain] Adjustments for New Accounting Pronouncements [Axis] MidCap Financial Services, LLC [Member] Represents MidCap Financial Services, LLC entity. Interest and other income Senior Credit Facility [Member] Represents senior credit facility. Credit facilities provide capital to borrowers without the need to structure a loan for each borrowing. us-gaap_CostsAndExpenses Total operating expenses Expenses Net proceeds from common stock issued under stock-based compensation plans Mundipharma [Member] Information pertaining to Mundipharma. Sale of common stock, net Proceeds from Issuance of Common Stock Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Domain] Difference between Revenue Guidance in Effect before and after Topic 606 [Member] bcrx_PercentageOfCarryingAmountInExcessOfFairValue1 Percentage of Carrying Amount in Excess of Fair Value Represents the percentage of carrying amount in excess of fair value. 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