Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Numerator:
Net income attributable to Biogen Inc.	$ 1,058.0	$ 448.5	$ 1,361.8	$ 858.7
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	145,900	149,700	146,500	150,800
Effect of dilutive securities:
Dilutive potential common shares	300	400	300	400
Shares used in calculating diluted earnings per share	146,200	150,100	146,800	151,200
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	200	200	200	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