Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator:
Net income attributable to Biogen Inc.	$ 591.6	$ 1,058.0	$ 979.5	$ 1,361.8
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	144,700	145,900	144,600	146,500
Effect of dilutive securities:
Dilutive potential common shares	800	300	800	300
Shares used in calculating diluted earnings per share	145,500	146,200	145,400	146,800
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	700	200	700	200
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100	100	100