Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2016

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted earnings per share are calculated as follows:

	For the Three Months Ended March 31,
(In millions)	2016		2015
Numerator:
Net income attributable to Biogen Inc.	$	970.9	$	822.5
Denominator:
Weighted average number of common shares outstanding	218.9		235.0
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.1
Time-vested restricted stock units	0.2		0.3
Market stock units	0.1		0.2
Dilutive potential common shares	0.4		0.6
Shares used in calculating diluted earnings per share	219.3		235.6