Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator:
Net income (loss) attributable to Biogen Inc.	$ (68.1)	$ 1,134.7	$ 911.4	$ 2,496.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	144,800	144,400	144,700	145,800
Effect of dilutive securities:
Dilutive potential common shares	0	400	800	400
Shares used in calculating diluted earnings per share	144,800	144,800	145,500	146,200
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0	300	700	300
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0	100	100	100