Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2020

Basic and diluted earnings per share are calculated as follows:


	For the Years Ended December 31,
(In millions)	2020		2019		2018
Numerator:
Net income attributable to Biogen Inc.	$	4,000.6	$	5,888.5	$	4,430.7
Denominator:
Weighted average number of common shares outstanding	160.9		187.1		204.9
Effect of dilutive securities:

Time-vested restricted stock units	0.2		0.2		0.3
Market stock units	0.1		0.1		0.1
Performance stock units settled in stock	0.1		—		—
Dilutive potential common shares	0.4		0.3		0.4
Shares used in calculating diluted earnings per share	161.3		187.4		205.3