Earnings per Share (Tables)

12 Months Ended

Dec. 31, 2025

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)	2025		2024		2023
Numerator:
Net income attributable to Biogen Inc.	$	1,292.9	$	1,632.2	$	1,161.1
Denominator:
Weighted-average number of common shares outstanding	146.5		145.6		144.7
Effect of dilutive securities:
Time-vested restricted stock units	0.5		0.3		0.7
Performance stock units settled in stock	0.1		—		0.2
Dilutive potential common shares	0.6		0.3		0.9
Shares used in calculating diluted earnings per share	147.1		145.9		145.6