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Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income attributable to Biogen Inc.	$ 410.2	$ 1,399.1
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	151.9	172.8
Effect of dilutive securities:
Dilutive potential common shares	0.4	0.3
Shares used in calculating diluted earnings per share	152.3	173.1
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.2	0.1
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.1	0.1

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