Earnings per Share

3 Months Ended

Mar. 31, 2020

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 March 31,
(In millions)	2020		2019
Numerator:
Net income attributable to Biogen Inc.	$	1,399.1	$	1,408.8
Denominator:
Weighted average number of common shares outstanding	172.8		196.6
Effect of dilutive securities:
Time-vested restricted stock units	0.1		0.3
Market stock units	0.1		0.1
Performance stock units settled in stock	0.1		—
Dilutive potential common shares	0.3		0.4
Shares used in calculating diluted earnings per share	173.1		197.0

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