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Earnings per Share (Tables) | 6 Months Ended
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Jun. 30, 2019
Earnings Per Share [Abstract] |
Basic and diluted earnings per share | Basic and diluted earnings per share are calculated as follows:
| For the Three Months Ended June 30, |  | For the Six Months Ended June 30,
(In millions) | 2019 |  | 2018 |  | 2019 |  | 2018
Numerator: |  |  |  |  |  |  |
Net income attributable to Biogen Inc. | $ | 1,494.1 |  |  | $ | 866.6 |  |  | $ | 2,902.9 |  |  | $ | 2,039.5 |
Denominator: |  |  |  |  |  |  |
Weighted average number of common shares outstanding | 190.3 |  |  | 207.1 |  |  | 193.4 |  |  | 209.2 |
Effect of dilutive securities: |  |  |  |  |  |  |
Stock options and employee stock purchase plan | — |  |  | — |  |  | — |  |  | — |
Time-vested restricted stock units | 0.1 |  |  | 0.1 |  |  | 0.2 |  |  | 0.2 |
Market stock units | — |  |  | 0.1 |  |  | 0.1 |  |  | 0.1 |
Performance stock units settled in stock | — |  |  | — |  |  | — |  |  | — |
Dilutive potential common shares | 0.1 |  |  | 0.2 |  |  | 0.3 |  |  | 0.3 |
Shares used in calculating diluted earnings per share | 190.4 |  |  | 207.3 |  |  | 193.7 |  |  | 209.5 |

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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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