Consolidated Statements of Equity - USD ($) $ in Millions	Total	Total Biogen Inc. shareholders' equity	Preferred stock	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Treasury stock	Noncontrolling interests
Beginning Balance at Dec. 31, 2012	$ (6,963.8)	$ (6,961.5)	$ 0.0	$ (0.1)	$ (3,854.5)	$ 55.3	$ (4,486.8)	$ (1,324.6)	$ (2.3)
Beginning Balance, shares at Dec. 31, 2012			0	(254,200,000)				(17,700,000)
Net income	1,862.3	1,862.3					1,862.3		0.0
Other comprehensive income, net of tax	27.6	27.6				27.6			0.0
Deconsolidation of noncontrolling interests	1.7	0.0							1.7
Repurchase of common stock, at cost	(400.3)	(400.3)						$ (400.3)
Repurchase of common stock, at cost, shares								(2,000,000)
Issuance of common stock under stock option and stock purchase plans	66.7	66.7		$ 0.0	66.7
Issuance of common stock under stock option and stock purchase plans, shares				800,000
Stock Issued During Period, Value, Stock Options Exercised	(89.7)	(89.7)		$ 0.0	(89.7)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				1,000,000
Compensation expense related to share-based payments	146.2	146.2			146.2
Tax benefit from share-based payments	45.9	45.9			45.9
Ending Balance at Dec. 31, 2013	(8,620.8)	(8,620.2)	$ 0.0	$ (0.1)	(4,023.6)	(27.7)	(6,349.1)	$ (1,724.9)	(0.6)
Ending Balance, shares at Dec. 31, 2013			0	(256,000,000)				(19,700,000)
Net income	2,941.6	2,934.8					2,934.8		6.8
Other comprehensive income, net of tax	31.8	31.8				31.8			0.0
Distribution to noncontrolling interests	(9.1)	0.0							(9.1)
Acquisition of noncontrolling interests	6.7	0.0							6.7
Repurchase of common stock, at cost	$ (886.8)	(886.8)						$ (886.8)
Repurchase of common stock, at cost, shares	(2,900,000)							(2,900,000)
Issuance of common stock under stock option and stock purchase plans	$ 54.9	54.9		$ 0.0	54.9
Issuance of common stock under stock option and stock purchase plans, shares				300,000
Stock Issued During Period, Value, Stock Options Exercised	(140.3)	(140.3)		$ 0.0	(140.3)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period				800,000
Compensation expense related to share-based payments	165.0	165.0			165.0
Tax benefit from share-based payments	93.0	93.0			93.0
Ending Balance at Dec. 31, 2014	(10,814.0)	(10,809.0)	$ 0.0	$ (0.1)	(4,196.2)	(59.5)	(9,283.9)	$ (2,611.7)	(5.0)
Ending Balance, shares at Dec. 31, 2014			0	(257,100,000)				(22,600,000)
Net income	3,593.2	3,547.0					3,547.0		46.2
Other comprehensive income, net of tax	(164.5)	(164.5)				(164.5)			0.0
Distribution to noncontrolling interests	(60.0)	0.0							(60.0)
Acquisition of noncontrolling interests	10.9	0.0							10.9
Repurchase of common stock, at cost	$ (5,000.0)	(5,000.0)						$ (5,000.0)
Repurchase of common stock, at cost, shares	(16,800,000)							(16,800,000)
Retirement of common stock pursuant to the 2015 Share Repurchase Program, at cost	$ 0.0	0.0		$ 0.0	4,377.5		622.5	$ 5,000.0
Retirement of common stock pursuant to the 2015 Share Repurchase Program, at cost, shares				16,800,000				16,800,000
Issuance of common stock under stock option and stock purchase plans	54.2	54.2		$ 0.0	54.2
Issuance of common stock under stock option and stock purchase plans, shares				300,000
Stock Issued During Period, Value, Stock Options Exercised	$ (125.1)	(125.1)		$ 0.0	(125.1)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	114,000			600,000
Compensation expense related to share-based payments	$ 183.2	183.2			183.2
Tax benefit from share-based payments	69.0	69.0			69.0
Ending Balance at Dec. 31, 2015	$ (9,374.9)	$ (9,372.8)	$ 0.0	$ (0.1)	$ 0.0	$ (224.0)	$ (12,208.4)	$ (2,611.7)	$ (2.1)
Ending Balance, shares at Dec. 31, 2015			0	(241,200,000)				(22,600,000)