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Earnings per Share (Tables) | 12 Months Ended
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Dec. 31, 2017
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) | 2017 |  | 2016 |  | 2015
Numerator: |  |  |  |  |
Net income attributable to Biogen Inc. | $ | 2,539.1 |  |  | $ | 3,702.8 |  |  | $ | 3,547.0 |
Denominator: |  |  |  |  |
Weighted average number of common shares outstanding | 212.6 |  |  | 218.4 |  |  | 230.7 |
Effect of dilutive securities: |  |  |  |  |
Stock options and employee stock purchase plan | 0.1 |  |  | 0.1 |  |  | 0.1 |
Time-vested restricted stock units | 0.2 |  |  | 0.2 |  |  | 0.3 |
Market stock units | 0.1 |  |  | 0.1 |  |  | 0.1 |
Dilutive potential common shares | 0.4 |  |  | 0.4 |  |  | 0.5 |
Shares used in calculating diluted earnings per share | 213.0 |  |  | 218.8 |  |  | 231.2 |

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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/2003/role/presentationRef -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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