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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, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | $ 2.4 | $ 1.6 | $ 2.9
Realized losses on sale of securities | (60.6) | (41.5) | (58.4)
Net change in expected credit gains (losses) | 1.9 | 4.8 | (5.0)
Fixed income securities, trading - gross realized gains | 1.2 | 1.0 | 0.2
Fixed income securities, trading, gross realizes (losses) | (10.0) | (3.5) | (1.8)
Privately-held investments net unrealized gains/(losses) | 26.8 | 65.9 | (113.9)
Catastrophe bonds | 0.0 | 0.1 | 0.2
Total net realized and unrealized investment (losses)/gains recorded in the consolidated statement of operations | (49.5) | 14.5 | (177.6)
Change in available for sale net unrealized gains/(losses): |  |  |
Available for sale investments | 34.1 | 126.2 | (391.7)
Income tax (expense)/benefit | (4.7) | (20.6) | 23.9
Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income | 29.4 | 105.6 | (367.8)
Privately-held investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities, trading - gross realized gains | 0.8 | 0.8 | 0.7
Fixed income securities, trading, gross realizes (losses) | (28.8) | 0.0 | (0.1)
Privately-held investments net unrealized gains/(losses) | 18.6 | (15.2) | (2.5)
Short-term investments — Available for sale |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | 0.8 | 0.6 | 1.0
Realized losses on sale of securities | (2.0) | (0.9) | (0.5)
Fixed income securities, trading - gross realized gains | 0.0 | 0.1 | 0.0
Fixed income securities, trading, gross realizes (losses) | (0.3) | (0.3) | 0.0
MVI |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | (0.2) | 0.2 | 0.0
Multi-Line Insurer |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.1 | 0.8 | (0.4)
Digital Re |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | $ (0.2) | $ 0.0 | $ 0.0

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