Represents interest reserve established as part of the royalty monetization.

No definition available.

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Namespace Prefix:	bcrx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the percentage of carrying amount in excess of fair value.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents private placement of senior secured notes.

+ References

No definition available.

+ Details

Name:	bcrx_PrivatePlacementOfSeniorSecuredNotes
Namespace Prefix:	bcrx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents transaction costs associated with royalty monetization.

+ References

No definition available.

+ Details

Name:	bcrx_TransactionCosts
Namespace Prefix:	bcrx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
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Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Paragraph 1
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Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual rate at which a foreign currency can be purchased or sold.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeForwardExchangeRate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of decrease in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value portion of notes payable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from amounts received from issuance of long-term debt that is wholly or partially secured by collateral. Excludes proceeds from tax exempt secured debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Name:	us-gaap_ProceedsFromIssuanceOfSecuredDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=bcrx_PhaRMANotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bcrx_CurrencyHedgeAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=bcrx_JPRRoyaltySubLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Period Type:

Note 4 - Royalty Monetization (Details Textual) $ in Thousands		9 Months Ended
	Mar. 09, 2011 USD ($)	Sep. 30, 2020 USD ($) ¥ / $	Sep. 30, 2019 USD ($)
Foreign Exchange Contract [Member] \| Not Designated as Hedging Instrument [Member]
Derivative, Loss on Derivative		$ 630	$ 532
PhaRMA Notes Member] \| Currency Hedge Agreement [Member]
Derivative, Forward Exchange Rate (in JPY per USD) \| ¥ / $		100
JPR Royalty Sub LLC [Member] \| PhaRMA Notes Member]
Private Placement of Senior Secured Notes	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage	14.00%
Percentage of Carrying Amount in Excess of Fair Value		3.00%
JPR Royalty Sub LLC [Member] \| PhaRMA Notes Member] \| Fair Value, Inputs, Level 2 [Member]
Notes Payable, Fair Value Disclosure		$ 30,000
JPR Royalty Sub LLC [Member] \| Royalty Monetization [Member]
Debt Instrument, Face Amount	$ 30,000
Proceeds from Issuance of Secured Debt	22,691
Transaction Costs	4,309
Interest Reserve	$ 3,000