Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Numerator:
Net income attributable to Biogen Inc.	$ 240.5	$ 393.4
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	146.1	145.2
Effect of dilutive securities:
Dilutive potential common shares	0.5	0.7
Shares used in calculating diluted earnings per share	146.6	145.9
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.5	0.5
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.0	0.2