Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income attributable to Biogen Inc	$ 1,172.9	$ 747.6
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	211.4	215.6
Effect of dilutive securities:
Dilutive potential common shares	0.3	0.3
Shares used in calculating diluted earnings per share	211.7	215.9
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	0.2	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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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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