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Earnings per Share | 12 Months Ended
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Dec. 31, 2017
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) | 2017 |  | 2016 |  | 2015
Numerator: |  |  |  |  |
Net income attributable to Biogen Inc. | $ | 2,539.1 |  |  | $ | 3,702.8 |  |  | $ | 3,547.0 |
Denominator: |  |  |  |  |
Weighted average number of common shares outstanding | 212.6 |  |  | 218.4 |  |  | 230.7 |
Effect of dilutive securities: |  |  |  |  |
Stock options and employee stock purchase plan | 0.1 |  |  | 0.1 |  |  | 0.1 |
Time-vested restricted stock units | 0.2 |  |  | 0.2 |  |  | 0.3 |
Market stock units | 0.1 |  |  | 0.1 |  |  | 0.1 |
Dilutive potential common shares | 0.4 |  |  | 0.4 |  |  | 0.5 |
Shares used in calculating diluted earnings per share | 213.0 |  |  | 218.8 |  |  | 231.2 |
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, 2017, 2016 and 2015 reflects, on a weighted average basis, the repurchase of 3.7 million shares, 0.7 million shares and 4.6 million shares, respectively, of our common stock under our share repurchase authorizations. The adjustments related to the spin-off of our hemophilia business did not have a material impact on the potentially dilutive securities to be considered in the calculation of diluted earnings per share of common stock.

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- Definition The entire disclosure for earnings per share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -URI http://asc.fasb.org/topic&trid=2144383 + Details
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