Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Numerator:
Net income attributable to Biogen Inc	$ 822,541us-gaap_NetIncomeLoss	$ 479,956us-gaap_NetIncomeLoss
Denominator:
Weighted average number of common shares outstanding	234,995,000	236,786,000
Effect of dilutive securities:
Dilutive potential common shares	600,000	1,000,000
Shares used in calculating diluted earnings per share	235,630,000	237,849,000
Stock options and employee stock purchase plan
Effect of dilutive securities:
Stock units	100,000	100,000
Time-vested restricted stock units
Effect of dilutive securities:
Stock units	300,000	600,000
Market stock units
Effect of dilutive securities:
Stock units	200,000	300,000

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