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v3.21.2

Earnings per Share (Details) - USD ($) shares in Millions, $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Numerator: |  |  |  |
Net income attributable to Biogen Inc. | $ 329.2 | $ 701.5 | $ 1,187.9 | $ 3,642.7
Weighted-average shares used in calculating: |  |  |  |
Weighted average number of common shares outstanding | 148.0 | 156.9 | 149.9 | 163.4
Effect of dilutive securities: |  |  |  |
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
Time-vested restricted stock units |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.4 | 0.1 | 0.2 | 0.1
Market stock units |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.1 | 0.1 | 0.1 | 0.1
Performance stock units settled in stock |  |  |  |
Effect of dilutive securities: |  |  |  |
Stock units | 0.1 | 0.1 | 0.1 | 0.1

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