Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income attributable to Biogen Inc.	$ 927,276	$ 714,506	$ 1,749,817	$ 1,194,462
Other comprehensive income:
Unrealized gains (losses) on securities available for sale, net of tax of $(145) and $(4,029) for the three months ended June 30, 2015 and 2014, respectively; and $621 and $(3,015) for the six months ended June 30, 2015 and 2014, respectively	(214)	(6,865)	1,085	(5,140)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of $(454) and $(260) for the three months ended June 30, 2015 and 2014, respectively; and $(42) and $5 for the six months ended June 30, 2015 and 2014, respectively	(96,159)	10,760	(8,913)	16,551
Unrealized gains (losses) on pension benefit obligation	2,851	(170)	4,109	646
Currency translation adjustment	63,035	(8,047)	(37,818)	(10,991)
Total other comprehensive income (loss), net of tax	(30,487)	(4,322)	(41,537)	1,066
Comprehensive income attributable to Biogen Inc.	896,789	710,184	1,708,280	1,195,528
Comprehensive income (loss) attributable to noncontrolling interests, net of tax	(2,266)	8,626	(4,533)	8,398
Comprehensive income	$ 894,523	$ 718,810	$ 1,703,747	$ 1,203,926