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Earnings per Share | 12 Months Ended
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Dec. 31, 2020
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) |  | 2020 |  | 2019 |  | 2018
Numerator: |  |  |  |  |  |
Net income attributable to Biogen Inc. |  | $ | 4,000.6 |  |  | $ | 5,888.5 |  |  | $ | 4,430.7 |
Denominator: |  |  |  |  |  |
Weighted average number of common shares outstanding |  | 160.9 |  |  | 187.1 |  |  | 204.9 |
Effect of dilutive securities: |  |  |  |  |  |
Time-vested restricted stock units |  | 0.2 |  |  | 0.2 |  |  | 0.3 |
Market stock units |  | 0.1 |  |  | 0.1 |  |  | 0.1 |
Performance stock units settled in stock |  | 0.1 |  |  | — |  |  | — |
Dilutive potential common shares |  | 0.4 |  |  | 0.3 |  |  | 0.4 |
Shares used in calculating diluted earnings per share |  | 161.3 |  |  | 187.4 |  |  | 205.3 |
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, 2020, 2019 and 2018, reflects the repurchase of approximately 22.4 million shares, 23.6 million shares and 14.8 million shares of our common stock, respectively, under our share repurchase programs. For additional information on our share repurchase programs, please read Note 13, Equity, to these consolidated financial statements.

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