Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income attributable to Biogen Inc.	$ 1,545.9	$ 1,444.4	$ 4,448.8	$ 3,483.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	184.0	201.4	190.3	206.6
Effect of dilutive securities:
Dilutive potential common shares	0.2	0.5	0.2	0.4
Shares used in calculating diluted earnings per share	184.2	201.9	190.5	207.0
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.0	0.0	0.0	0.0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.1	0.4	0.1	0.3
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	0.0	0.0	0.0	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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