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Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Numerator: |  |  |  |
Net income attributable to Biogen Inc. | $ 1,494.1 | $ 866.6 | $ 2,902.9 | $ 2,039.5
Weighted-average shares used in calculating: |  |  |  |
Weighted average number of common shares outstanding | 190.3 | 207.1 | 193.4 | 209.2
Effect of dilutive securities: |  |  |  |
Dilutive potential common shares | 0.1 | 0.2 | 0.3 | 0.3
Shares used in calculating diluted earnings per share | 190.4 | 207.3 | 193.7 | 209.5
Stock options and employee stock purchase plan |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.0 | 0.0 | 0.0 | 0.0
Time-vested restricted stock units |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.1 | 0.1 | 0.2 | 0.2
Market stock units |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.0 | 0.1 | 0.1 | 0.1
Performance stock units settled in stock |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.0 | 0.0 | 0.0 | 0.0

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