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Earnings per Share | 9 Months Ended
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Sep. 30, 2019
Earnings Per Share [Abstract] |
Earnings per Share | Earnings per Share Basic and diluted earnings per share are calculated as follows:
| For the Three Months Ended September 30, |  | For the Nine Months Ended September 30,
(In millions) | 2019 |  | 2018 |  | 2019 |  | 2018
Numerator: |  |  |  |  |  |  |
Net income attributable to Biogen Inc. | $ | 1,545.9 |  |  | $ | 1,444.4 |  |  | $ | 4,448.8 |  |  | $ | 3,483.9 |
Denominator: |  |  |  |  |  |  |
Weighted average number of common shares outstanding | 184.0 |  |  | 201.4 |  |  | 190.3 |  |  | 206.6 |
Effect of dilutive securities: |  |  |  |  |  |  |
Stock options and employee stock purchase plan | — |  |  | — |  |  | — |  |  | — |
Time-vested restricted stock units | 0.1 |  |  | 0.4 |  |  | 0.1 |  |  | 0.3 |
Market stock units | 0.1 |  |  | 0.1 |  |  | 0.1 |  |  | 0.1 |
Performance stock units settled in stock | — |  |  | — |  |  | — |  |  | — |
Dilutive potential common shares | 0.2 |  |  | 0.5 |  |  | 0.2 |  |  | 0.4 |
Shares used in calculating diluted earnings per share | 184.2 |  |  | 201.9 |  |  | 190.5 |  |  | 207.0 |
Amounts excluded from the calculation of net income per diluted share because their effects were anti-dilutive were insignificant.

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