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Earnings per Share (Tables) | 12 Months Ended
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Dec. 31, 2020
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) |  | 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 |

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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://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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