Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2018

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)	2018		2017
Numerator:
Net income attributable to Biogen Inc.	$	1,172.9	$	747.6
Denominator:
Weighted-average number of common shares outstanding	211.4		215.6
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		—
Time-vested restricted stock units	0.2		0.2
Market stock units	0.1		0.1
Performance stock units settled in stock	—		—
Dilutive potential common shares	0.3		0.3
Shares used in calculating diluted earnings per share	211.7		215.9