Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator:
Net income (loss) attributable to Biogen Inc.	$ 583.6	$ 591.6	$ 977.0	$ 979.5
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	145,600	144,700	145,400	144,600
Effect of dilutive securities:
Dilutive potential common shares	300	800	500	800
Shares used in calculating diluted earnings per share	145,900	145,500	145,900	145,400
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	200	700	400	700
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100	100	100