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Earnings per Share (Tables) | 9 Months Ended
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Sep. 30, 2021
Earnings Per Share [Abstract] |
Basic and diluted earnings per share | Basic and diluted shares outstanding used in our earnings per share calculation are calculated as follows:
|  | For the Three Months Ended September 30, |  | For the Nine Months Ended September 30,
(In millions) |  | 2021 |  | 2020 |  | 2021 |  | 2020
Numerator: |  |  |  |  |  |  |  |
Net income attributable to Biogen Inc. |  | $ | 329.2 |  |  | $ | 701.5 |  |  | $ | 1,187.9 |  |  | $ | 3,642.7 |
Denominator: |  |  |  |  |  |  |  |
Weighted average number of common shares outstanding |  | 148.0 |  |  | 156.9 |  |  | 149.9 |  |  | 163.4 |
Effect of dilutive securities: |  |  |  |  |  |  |  |
Time-vested restricted stock units |  | 0.4 |  |  | 0.1 |  |  | 0.2 |  |  | 0.1 |
Market stock units |  | 0.1 |  |  | 0.1 |  |  | 0.1 |  |  | 0.1 |
Performance stock units settled in stock |  | 0.1 |  |  | 0.1 |  |  | 0.1 |  |  | 0.1 |
Dilutive potential common shares |  | 0.6 |  |  | 0.3 |  |  | 0.4 |  |  | 0.3 |
Shares used in calculating diluted earnings per share |  | 148.6 |  |  | 157.2 |  |  | 150.3 |  |  | 163.7 |

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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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