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Earnings per Share (Tables) | 3 Months Ended
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Mar. 31, 2015
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 March 31,
(In millions) | 2015 |  | 2014
Numerator: |  |  |
Net income attributable to Biogen Inc. | $ | 822.5 |  |  | $ | 480.0 |
Denominator: |  |  |
Weighted average number of common shares outstanding | 235.0 |  |  | 236.8 |
Effect of dilutive securities: |  |  |
Stock options and employee stock purchase plan | 0.1 |  |  | 0.1 |
Time-vested restricted stock units | 0.3 |  |  | 0.6 |
Market stock units | 0.2 |  |  | 0.3 |
Dilutive potential common shares | 0.6 |  |  | 1.0 |
Shares used in calculating diluted earnings per share | 235.6 |  |  | 237.8 |

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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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