Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 965,622	$ 856,858	$ 2,715,439	$ 2,051,320
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $(1,320) and $(6) for the three months ended September 30, 2015 and 2014, respectively; and $(699) and $(3,021) for the nine months ended September 30, 2015 and 2014, respectively	(2,239)	12	(1,154)	(5,127)
Unrealized gains (losses) on cash flow hedges, net of tax of $(187) and $302 for the three months ended September 30, 2015 and 2014, respectively; and $(229) and $307 for the nine months ended September 30, 2015 and 2014, respectively	(31,171)	48,242	(40,084)	64,793
Unrealized gains (losses) on pension benefit obligation	523	691	4,632	1,338
Currency translation adjustment	(23,504)	(60,254)	(61,322)	(71,246)
Total other comprehensive income (loss), net of tax	(56,391)	(11,309)	(97,928)	(10,242)
Comprehensive income attributable to Biogen Inc.	909,231	845,549	2,617,511	2,041,078
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	53,586	(738)	49,053	7,660
Comprehensive income	$ 962,817	$ 844,811	$ 2,666,564	$ 2,048,738