Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2021

Earnings Per Share [Abstract]

Basic and diluted earnings per share

Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows:

	For the Years Ended December 31,
(In millions)	2021		2020		2019
Numerator:
Net income attributable to Biogen Inc.	$	1,556.1	$	4,000.6	$	5,888.5
Denominator:
Weighted average number of common shares outstanding	149.1		160.9		187.1
Effect of dilutive securities:
Time-vested restricted stock units	0.3		0.2		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	0.5		0.4		0.3
Shares used in calculating diluted earnings per share	149.6		161.3		187.4