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Investments - Summary of Investment Income (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income $ 329.5 $ 287.3 $ 198.4
Gain (loss) on investments 52.6 75.9 5.0
Investment expenses (11.5) (11.6) (10.3)
Net investment income 318.0 275.7 188.1
Fixed income securities — Available for sale      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 161.5 115.7 95.6
Fixed income securities — Trading      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 88.2 98.9 57.0
Short-term investments — Available for sale      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 4.5 5.3 0.6
Short-term investments — Trading      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 0.1 0.3 0.1
Fixed term deposits (included in cash and cash equivalents)      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 45.3 39.9 6.6
Catastrophe bonds — Trading      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 0.0 0.2 0.3
Privately-held investments — Available for sale      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 1.2 0.1 0.0
Privately-held investments — Trading      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Investment income 34.9 44.7 24.3
Other investments, at fair value      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Gain (loss) on investments $ 6.2 $ 17.8 $ 13.9