Earnings per Share

6 Months Ended

Jun. 30, 2018

Earnings Per Share [Abstract]

Earnings per Share

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)	2018		2017		2018		2017
Numerator:
Net income attributable to Biogen Inc.	$	866.6	$	862.8	$	2,039.5	$	1,610.4
Denominator:
Weighted-average number of common shares outstanding	207.1		211.9		209.2		213.7
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		0.1		—		—
Time-vested restricted stock units	0.1		0.1		0.2		0.2
Market stock units	0.1		0.1		0.1		0.1
Performance stock units settled in stock	—		—		—		—
Dilutive potential common shares	0.2		0.3		0.3		0.3
Shares used in calculating diluted earnings per share	207.3		212.2		209.5		214.0

Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.