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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, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | $ 1.6 | $ 2.9 | $ 22.7
Fixed income securities - gross realizes (losses) | (41.5) | (58.4) | (3.6)
Net change in expected credit gains (losses) | 4.8 | (5.0) | (2.5)
Fixed income securities, trading - gross realized gains | 1.0 | 0.2 | 12.2
Fixed income securities, trading, gross realizes (losses) | (3.5) | (1.8) | (2.0)
Privately-held investments net unrealized gains/(losses) | 65.9 | (113.9) | (23.4)
Catastrophe bonds | 0.1 | 0.2 | (0.8)
Total net realized and unrealized investment gains/(losses) recorded in the consolidated statement of operations | 14.5 | (177.6) | 8.8
Change in available for sale net unrealized gains/(losses): |  |  |
Available for sale investments | 126.2 | (391.7) | (157.6)
Income tax (expense)/benefit | (20.6) | 23.9 | (0.3)
Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income | 105.6 | (367.8) | (157.9)
Privately-held investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities, trading - gross realized gains | 0.8 | 0.7 | 0.6
Fixed income securities, trading, gross realizes (losses) | 0.0 | (0.1) | (13.8)
Privately-held investments net unrealized gains/(losses) | 15.2 | 2.5 | (18.1)
Short-term investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Fixed income securities - gross realized gains | 0.6 | 1.0 | 2.0
Fixed income securities - gross realizes (losses) | (0.9) | (0.5) | (0.8)
Fixed income securities, trading - gross realized gains | 0.1 | 0.0 | 0.1
Fixed income securities, trading, gross realizes (losses) | (0.3) | 0.0 | (0.3)
MVI |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | 0.2 | 0.0 | 0.1
Multi-Line Insurer |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |
Income/(loss) from equity method investment | $ 0.8 | $ (0.4) | $ 0.2

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