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Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2015 | Mar. 31, 2014
Numerator: |  |
Net income attributable to Biogen Inc | $ 822,541 us-gaap_NetIncomeLoss | $ 479,956 us-gaap_NetIncomeLoss
Denominator: |  |
Weighted average number of common shares outstanding | 234,995,000 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 236,786,000 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Effect of dilutive securities: |  |
Dilutive potential common shares | 600,000 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment | 1,000,000 us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
Shares used in calculating diluted earnings per share | 235,630,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 237,849,000 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Stock options and employee stock purchase plan |  |
Effect of dilutive securities: |  |
Stock units | 100,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = biib_StockOptionsAndEmployeeStockPurchasePlanMember | 100,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = biib_StockOptionsAndEmployeeStockPurchasePlanMember
Time-vested restricted stock units |  |
Effect of dilutive securities: |  |
Stock units | 300,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = biib_TimeVestedRestrictedStockUnitsMember | 600,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = biib_TimeVestedRestrictedStockUnitsMember
Market stock units |  |
Effect of dilutive securities: |  |
Stock units | 200,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = biib_MarketStockUnitsMember | 300,000 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = biib_MarketStockUnitsMember

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- Definition Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 28A -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1500-109256 + Details
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- Definition The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Other Comprehensive Income -URI http://asc.fasb.org/extlink&oid=6519514 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Net Income -URI http://asc.fasb.org/extlink&oid=6518256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.19) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.18) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.22) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 5 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 20 -Article 9 + Details
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