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Interim Financial Information - Basis of Presentation (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Jun. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Business And Organization [Line Items] |  |  |  |  |
Out-of-period adjustment |  | $ 10.6 |  |  |
Net income (loss) | $ 126.4 |  | $ 38.0 | $ 186.4 | $ 125.9
Diluted earnings (loss) per common share (in dollars per share) | $ 6.83 |  | $ 2.04 | $ 10.08 | $ 6.77
Restatement Adjustment |  |  |  |  |
Business And Organization [Line Items] |  |  |  |  |
Net income (loss) |  | $ (9.7) |  |  |
Diluted earnings (loss) per common share (in dollars per share) |  | $ (0.53) |  |  |

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