Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income attributable to Biogen Inc.	$ 1,408.8	$ 1,172.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	196.6	211.4
Effect of dilutive securities:
Dilutive potential common shares	0.4	0.3
Shares used in calculating diluted earnings per share	197.0	211.7
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.0	0.0
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	0.3	0.2
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	0.0	0.0

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