Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income attributable to Biogen Inc.	$ 701.5	$ 1,545.9	$ 3,642.7	$ 4,448.8
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	156.9	184.0	163.4	190.3
Effect of dilutive securities:
Dilutive potential common shares	0.3	0.2	0.3	0.2
Shares used in calculating diluted earnings per share	157.2	184.2	163.7	190.5
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.1	0.1	0.1	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	0.0	0.1	0.0

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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