Earnings per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Numerator:
Net income attributable to Biogen Inc.	$ 1,632.2	$ 1,161.1	$ 3,046.9
Denominator:
Weighted-average number of common shares outstanding (in shares)	145.6	144.7	145.3
Effect of dilutive securities:
Dilutive potential common shares (in shares)	0.3	0.9	0.7
Shares used in calculating diluted earnings per share (in shares)	145.9	145.6	146.0
Time-vested restricted stock units
Effect of dilutive securities:
Dilutive securities (in shares)	0.3	0.7	0.5
Market stock units
Effect of dilutive securities:
Dilutive securities (in shares)	0.0	0.0	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Dilutive securities (in shares)	0.0	0.2	0.1