Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator:
Net income attributable to Biogen Inc.	$ 1,134.7	$ 329.2	$ 2,496.5	$ 1,187.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	144,400	148,000	145,800	149,900
Effect of dilutive securities:
Dilutive potential common shares	400	600	400	400
Shares used in calculating diluted earnings per share	144,800	148,600	146,200	150,300
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	300	400	300	200
Market stock units
Effect of dilutive securities:
Stock units	0	100	0	100
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100	100	100