Earnings per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 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 (in shares)	146.5	145.6	144.7
Effect of dilutive securities:
Dilutive potential common shares (in shares)	0.6	0.3	0.9
Shares used in calculating diluted earnings per share (in shares)	147.1	145.9	145.6
Time-vested restricted stock units
Effect of dilutive securities:
Dilutive securities (in shares)	0.5	0.3	0.7
Performance stock units settled in stock
Effect of dilutive securities:
Dilutive securities (in shares)	0.1	0.0	0.2