Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2015

Earnings Per Share [Abstract]

Basic and diluted earnings per share

	For the Years Ended December 31,
(In millions)	2015		2014		2013
Numerator:
Net income attributable to Biogen Inc.	$	3,547.0	$	2,934.8	$	1,862.3
Denominator:
Weighted average number of common shares outstanding	230.7		236.4		236.9
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.1		0.3
Time-vested restricted stock units	0.3		0.5		0.8
Market stock units	0.1		0.2		0.3
Dilutive potential common shares	0.5		0.8		1.4
Shares used in calculating diluted earnings per share	231.2		237.2		238.3