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Earnings per Share | 12 Months Ended
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Dec. 31, 2015
Earnings Per Share [Abstract] |
Earnings per Share | Earnings per Share Basic and diluted earnings per share are calculated as follows:
| For the Years Ended December 31,
(In millions) | 2015 |  | 2014 |  | 2013
Numerator: |  |  |  |  |
Net income attributable to Biogen Inc. | $ | 3,547.0 |  |  | $ | 2,934.8 |  |  | $ | 1,862.3 |
Denominator: |  |  |  |  |
Weighted average number of common shares outstanding | 230.7 |  |  | 236.4 |  |  | 236.9 |
Effect of dilutive securities: |  |  |  |  |
Stock options and employee stock purchase plan | 0.1 |  |  | 0.1 |  |  | 0.3 |
Time-vested restricted stock units | 0.3 |  |  | 0.5 |  |  | 0.8 |
Market stock units | 0.1 |  |  | 0.2 |  |  | 0.3 |
Dilutive potential common shares | 0.5 |  |  | 0.8 |  |  | 1.4 |
Shares used in calculating diluted earnings per share | 231.2 |  |  | 237.2 |  |  | 238.3 |
Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant. Earnings per share for the years ended December 31, 2015, 2014 and 2013, reflects, on a weighted average basis, the repurchase of 4.6 million shares, 1.0 million shares and 0.9 million shares, respectively, of our common stock under our share repurchase authorizations.

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