Earnings per Share (Tables)

3 Months Ended

Mar. 31, 2020

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)	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