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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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