Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Numerator:
Net income (loss) attributable to Biogen Inc.	$ 388.5	$ (68.1)	$ 1,365.5	$ 911.4
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	145,700	144,800	145,500	144,700
Effect of dilutive securities:
Dilutive potential common shares	400	0	500	800
Shares used in calculating diluted earnings per share	146,100	144,800	146,000	145,500
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	300	0	400	700
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	0	100	100