Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net income (loss) attributable to Biogen Inc.	$ 393.4	$ 387.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	145.2	144.4
Effect of dilutive securities:
Dilutive potential common shares	0.7	0.8
Shares used in calculating diluted earnings per share	145.9	145.2
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.5	0.6
Market stock units
Effect of dilutive securities:
Stock units	0.0	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.2	0.1