Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income attributable to Biogen Inc.	$ 1,542.1	$ 1,494.1	$ 2,941.2	$ 2,902.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	160.6	190.3	166.7	193.4
Effect of dilutive securities:
Dilutive potential common shares	0.3	0.1	0.3	0.3
Shares used in calculating diluted earnings per share	160.9	190.4	167.0	193.7
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.1	0.1	0.1	0.2
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.0	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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