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Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Net Investment Income [Line Items]      
Total investment income $ 196.6 $ 126.5 $ 84.8
Third-party investment expenses (2.4) (2.1) (2.3)
Net investment income, pre-tax 194.2 124.4 82.5
Fixed maturity investments      
Net Investment Income [Line Items]      
Total investment income 64.4 41.4 28.8
Short-term investments      
Net Investment Income [Line Items]      
Total investment income 52.0 16.4 0.6
Common equity securities      
Net Investment Income [Line Items]      
Total investment income 4.1 1.6 0.1
Other long-term investments      
Net Investment Income [Line Items]      
Total investment income 76.1 68.1 56.3
Amount attributable to TPC Providers      
Net Investment Income [Line Items]      
Amount attributable to TPC Providers $ 0.0 $ (1.0) $ (1.0)