Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income attributable to Biogen Inc.	$ 1,399.1	$ 1,408.8
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	172.8	196.6
Effect of dilutive securities:
Dilutive potential common shares	0.3	0.4
Shares used in calculating diluted earnings per share	173.1	197.0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.1	0.3
Market stock units
Effect of dilutive securities:
Stock units	0.1	0.1
Performance stock units settled in stock
Effect of dilutive securities:
Stock units	0.1	0.0

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