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Earnings per Share | 3 Months Ended
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Mar. 31, 2020
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 March 31,
(In millions) |  | 2020 |  | 2019
Numerator: |  |  |  |
Net income attributable to Biogen Inc. |  | $ | 1,399.1 |  |  | $ | 1,408.8 |
Denominator: |  |  |  |
Weighted average number of common shares outstanding |  | 172.8 |  |  | 196.6 |
Effect of dilutive securities: |  |  |  |
Time-vested restricted stock units |  | 0.1 |  |  | 0.3 |
Market stock units |  | 0.1 |  |  | 0.1 |
Performance stock units settled in stock |  | 0.1 |  |  | — |
Dilutive potential common shares |  | 0.3 |  |  | 0.4 |
Shares used in calculating diluted earnings per share |  | 173.1 |  |  | 197.0 |
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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