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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 | 6 Months Ended
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Jun. 30, 2023 | Jun. 30, 2022
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Fixed income securities - gross realized gains | $ 0.4 | $ 1.4
Fixed income securities - gross realizes (losses) | (6.9) | (29.9)
Net change in expected credit gains (losses) | 3.7 | (7.7)
Fixed income securities, trading - gross realized gains | 0.3 | 0.0
Fixed income securities, trading, gross realizes (losses) | (1.1) | (0.2)
Privately-held investments net unrealized gains/(losses) | 31.0 | (89.6)
Catastrophe bonds | (0.6) | (0.1)
Total net realized and unrealized investment gains/(losses) recorded in the consolidated statement of operations | 18.0 | (126.4)
Change in available for sale net unrealized gains/(losses): |  |
Fixed income securities gains/(losses) | 12.3 | (316.0)
Income tax (expense)/benefit | (1.6) | 29.5
Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income | 10.7 | (286.5)
Cash and cash equivalents |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Fixed income securities - gross realized gains | 0.1 | 0.9
Fixed income securities - gross realizes (losses) | (0.1) | (0.2)
Fixed income securities, trading - gross realized gains | 0.1 | 0.0
Fixed income securities, trading, gross realizes (losses) | (0.2) | 0.0
Privately-held investments |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Fixed income securities, trading - gross realized gains | 0.1 | 0.2
Privately-held investments net unrealized gains/(losses) | 9.0 | 1.3
Short-term investments |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Fixed income securities - gross realizes (losses) | (0.2) | 0.0
Multi-Line Insurer |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |
Income/(loss) from equity method investment | $ 0.4 | $ 0.1

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