Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income attributable to Biogen Inc	$ 747.6	$ 970.9
Weighted-average shares used in calculating:
Weighted average number of common shares outstanding	215.6	218.9
Effect of dilutive securities:
Dilutive potential common shares	0.3	0.4
Shares used in calculating diluted earnings per share	215.9	219.3
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	0.0	0.1
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

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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 portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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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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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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