Earnings per Share (Tables)

9 Months Ended

Sep. 30, 2019

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)	2019		2018		2019		2018
Numerator:
Net income attributable to Biogen Inc.	$	1,545.9	$	1,444.4	$	4,448.8	$	3,483.9
Denominator:
Weighted average number of common shares outstanding	184.0		201.4		190.3		206.6
Effect of dilutive securities:
Stock options and employee stock purchase plan	—		—		—		—
Time-vested restricted stock units	0.1		0.4		0.1		0.3
Market stock units	0.1		0.1		0.1		0.1
Performance stock units settled in stock	—		—		—		—
Dilutive potential common shares	0.2		0.5		0.2		0.4
Shares used in calculating diluted earnings per share	184.2		201.9		190.5		207.0