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Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | $ 2.9 | $ 22.7 | $ 68.8
Fixed income securities - gross realizes (losses) | (58.4) | (3.6) | (1.8)
Net change in expected credit gains (losses) | (5.0) | (2.5) | 0.4
Fixed income securities, trading - gross realized gains | 0.2 | 12.2 | 18.2
Fixed income securities, trading, gross realizes (losses) | (1.8) | (2.0) | (3.8)
Privately-held investments net unrealized gains/(losses) | (113.9) | (23.4) | 0.0
Catastrophe bonds | 0.2 | (0.8) | 0.0
Total net realized and unrealized investment (losses)/gains recorded in the consolidated statement of operations | (177.6) | 8.8 | 71.1
Change in available for sale net unrealized (losses)/gains: |  |  |
Fixed income securities | (391.7) | (157.6) | 108.5
Income tax benefit/(expense) | 23.9 | (0.3) | (0.5)
Total change in net unrealized (losses)/gains, net of taxes recorded in other comprehensive income | (367.8) | (157.9) | 108.0
Cash and cash equivalents |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | 1.0 | 2.0 | 0.5
Fixed income securities - gross realizes (losses) | (0.5) | (0.8) | (0.4)
Fixed income securities, trading - gross realized gains | 0.0 | 0.1 | 0.2
Fixed income securities, trading, gross realizes (losses) | 0.0 | (0.3) | (0.3)
Privately-held investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities, trading - gross realized gains | 0.7 | 0.6 | 0.0
Fixed income securities, trading, gross realizes (losses) | (0.1) | (13.8) | 0.0
Privately-held investments net unrealized gains/(losses) | 2.5 | (18.1) | 20.4
MVI |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.0 | 0.1 | (0.4)
Multi-Line Insurer |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | (0.4) | 0.2 | 0.0
Digital Re |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.0 | 0.0 | (0.3)
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Bene |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Gross realized gain on sale of equity method investment | 0.0 | 0.0 | 1.8
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Crop Re |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Gross realized gain on sale of equity method investment | $ 0.0 | $ 0.0 | $ 8.6

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