Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)	$ (14,986)us-gaap_OtherNonoperatingIncomeExpense	$ (5,601)us-gaap_OtherNonoperatingIncomeExpense
Income tax benefit (expense)	281,881us-gaap_IncomeTaxExpenseBenefit	178,414us-gaap_IncomeTaxExpenseBenefit
Product, net	2,172,322us-gaap_SalesRevenueGoodsNet	1,742,765us-gaap_SalesRevenueGoodsNet
Net income attributable to Biogen Inc.	822,541us-gaap_NetIncomeLoss	479,956us-gaap_NetIncomeLoss
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to Biogen Inc.	34,700	(4,200)
Accumulated Net Unrealized Investment Gain (Loss) \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income (expense)	(100)	100
Income tax benefit (expense)	0	0
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges \| Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit (expense)	(200)	400
Product, net	$ 35,000	$ (4,700)

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts.

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