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Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net income attributable to Biogen Inc.	$ 329.2	$ 701.5	$ 1,187.9	$ 3,642.7
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	148.0	156.9	149.9	163.4
Effect of dilutive securities:
Dilutive potential common shares	0.6	0.3	0.4	0.3
Shares used in calculating diluted earnings per share	148.6	157.2	150.3	163.7
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.4	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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