Earnings per Share (Tables)

9 Months Ended

Sep. 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 September 30,				For the Nine Months Ended September 30,
(In millions)	2020		2019		2020		2019
Numerator:
Net income attributable to Biogen Inc.	$	701.5	$	1,545.9	$	3,642.7	$	4,448.8
Denominator:
Weighted average number of common shares outstanding	156.9		184.0		163.4		190.3
Effect of dilutive securities:
Time-vested restricted stock units	0.1		0.1		0.1		0.1
Market stock units	0.1		0.1		0.1		0.1
Performance stock units settled in stock	0.1		—		0.1		—
Dilutive potential common shares	0.3		0.2		0.3		0.2
Shares used in calculating diluted earnings per share	157.2		184.2		163.7		190.5