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Earnings per Share (Tables) | 12 Months Ended
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Dec. 31, 2019
Earnings Per Share [Abstract] |
Basic and diluted earnings per share | Basic and diluted earnings per share are calculated as follows:
| For the Years Ended December 31,
(In millions) | 2019 |  | 2018 |  | 2017
Numerator: |  |  |  |  |
Net income attributable to Biogen Inc. | $ | 5,888.5 |  |  | $ | 4,430.7 |  |  | $ | 2,539.1 |
Denominator: |  |  |  |  |
Weighted average number of common shares outstanding | 187.1 |  |  | 204.9 |  |  | 212.6 |
Effect of dilutive securities: |  |  |  |  |
Stock options and employee stock purchase plan | — |  |  | — |  |  | 0.1 |
Time-vested restricted stock units | 0.2 |  |  | 0.3 |  |  | 0.2 |
Market stock units | 0.1 |  |  | 0.1 |  |  | 0.1 |
Performance stock units settled in stock | — |  |  | — |  |  | — |
Dilutive potential common shares | 0.3 |  |  | 0.4 |  |  | 0.4 |
Shares used in calculating diluted earnings per share | 187.4 |  |  | 205.3 |  |  | 213.0 |

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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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