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Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net income attributable to Biogen Inc.	$ 448.5	$ 1,542.1	$ 858.7	$ 2,941.2
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	149.7	160.6	150.8	166.7
Effect of dilutive securities:
Dilutive potential common shares	0.4	0.3	0.4	0.3
Shares used in calculating diluted earnings per share	150.1	160.9	151.2	167.0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.2	0.1	0.2	0.1
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.1	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.1	0.1	0.1	0.1

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