Earnings per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Numerator:
Net income attributable to Biogen Inc.	$ 303.8	$ 410.2
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	147,100	151,900
Effect of dilutive securities:
Dilutive potential common shares	500	400
Shares used in calculating diluted earnings per share	147,600	152,300
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	300	200
Market stock units
Effect of dilutive securities:
Stock units	100	100
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	100	100

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