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Earnings per Share (Details) - USD ($) shares in Thousands, $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Numerator: |  |  |  |
Net income attributable to Biogen Inc | $ 927,276 | $ 714,506 | $ 1,749,817 | $ 1,194,462
Weighted-average shares used in calculating: |  |  |  |
Weighted average number of common shares outstanding | 235,286 | 236,661 | 235,122 | 236,729
Effect of dilutive securities: |  |  |  |
Dilutive potential common shares | 400 | 700 | 600 | 900
Shares used in calculating diluted earnings per share | 235,718 | 237,401 | 235,671 | 237,634
Stock options and employee stock purchase plan |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 100 | 100 | 100 | 100
Time-vested restricted stock units |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 200 | 400 | 300 | 500
Market stock units |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 100 | 200 | 200 | 300

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