Earnings per Share (Tables)

6 Months Ended

Jun. 30, 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 June 30,				For the Six Months Ended June 30,
(In millions)	2020		2019		2020		2019
Numerator:
Net income attributable to Biogen Inc.	$	1,542.1	$	1,494.1	$	2,941.2	$	2,902.9
Denominator:
Weighted average number of common shares outstanding	160.6		190.3		166.7		193.4
Effect of dilutive securities:
Time-vested restricted stock units	0.1		0.1		0.1		0.2
Market stock units	0.1		—		0.1		0.1
Performance stock units settled in stock	0.1		—		0.1		—
Dilutive potential common shares	0.3		0.1		0.3		0.3
Shares used in calculating diluted earnings per share	160.9		190.4		167.0		193.7