2.2.0.7 false Litigation (Details) (USD $) 0618 - Disclosure - Litigation (Details) true false In Millions false false 1 USD false false USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 2 USD false false USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 Pure Standard http://www.xbrl.org/2003/instance pure xbrli 0 USDEPS Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 $ false 3 USD false false USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ 3 1 biib_LitigationTextualsAbstract biib false na duration Litigation textuals abstract. false false false false false true false false false false false verboselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string Litigation textuals abstract. false 4 2 biib_NoticeOfAssessmentUnderConsiderationForCorporateExciseTax biib false debit duration Notice Of Assessment Under Consideration For Corporate Excise Tax. false false false false false false false false false false false verboselabel false 1 true true false false 103500000 103.5 false false false 2 false false false false 0 0 false false false 3 true true false false 38900000 38.9 false false false xbrli:monetaryItemType monetary Notice Of Assessment Under Consideration For Corporate Excise Tax. No authoritative reference available. false 5 2 biib_EstimatedRoyaltiesAndInterest biib false credit duration Estimated Royalties and interest based on a stipulated royalty rate if awarded in connection with RITUXAN. false false false false false false false false false false false verboselabel false 1 false false false false 0 0 false false false 2 true true false false 100000000 100 false false false 3 false false false false 0 0 false false false xbrli:monetaryItemType monetary Estimated Royalties and interest based on a stipulated royalty rate if awarded in connection with RITUXAN. No authoritative reference available. false 6 2 biib_StipulatedRoyaltyRate biib false na duration Stipulated royalty rate. false false false false false false false false false false false verboselabel false 1 false false false false 0 0 false false false 2 false true false false 0.005 0.005 false false false 3 false false false false 0 0 false false false us-types:percentItemType pure Stipulated royalty rate. No authoritative reference available. false 3 4 false HundredThousands UnKnown UnKnown false true