Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2017

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)	2017		2016
Numerator:
Net income attributable to Biogen Inc.	$	747.6	$	970.9
Denominator:
Weighted-average number of common shares outstanding	215.6		218.9
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		0.1
Time-vested restricted stock units	0.2		0.2
Market stock units	0.1		0.1
Dilutive potential common shares	0.3		0.4
Shares used in calculating diluted earnings per share	215.9		219.3