Earnings per Share (Tables)

6 Months Ended

Jun. 30, 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 June 30,				For the Six Months Ended June 30,
(In millions)	2017		2016		2017		2016
Numerator:
Net income attributable to Biogen Inc.	$	862.8	$	1,049.8	$	1,610.4	$	2,020.7
Denominator:
Weighted-average number of common shares outstanding	211.9		219.1		213.7		219.0
Effect of dilutive securities:
Stock options and employee stock purchase plan	0.1		0.1		—		0.1
Time-vested restricted stock units	0.1		0.1		0.2		0.1
Market stock units	0.1		0.1		0.1		0.1
Dilutive potential common shares	0.3		0.3		0.3		0.3
Shares used in calculating diluted earnings per share	212.2		219.4		214.0		219.3