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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Schedule of Invested Assets [Line Items]      
Amortized Cost $ 3,016.4 $ 4,203.2  
Allowance for Credit Losses 2.8 2.5 $ 6.6
Fair Value 2,675.5 4,223.3  
Fixed maturities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 3,016.4 4,203.2  
Gross Unrealized Gains 2.1 63.4  
Gross Unrealized Losses 340.2 40.8  
Allowance for Credit Losses 2.8 2.5  
Fair Value 2,675.5 4,223.3  
Fixed maturities | U.S. Governments      
Schedule of Invested Assets [Line Items]      
Amortized Cost 410.9 422.7  
Gross Unrealized Gains 0.0 5.5  
Gross Unrealized Losses 30.2 3.2  
Allowance for Credit Losses 0.0 0.0  
Fair Value 380.7 425.0  
Fixed maturities | Foreign Governments      
Schedule of Invested Assets [Line Items]      
Amortized Cost 35.6 234.7  
Gross Unrealized Gains 0.3 2.2  
Gross Unrealized Losses 6.7 3.9  
Allowance for Credit Losses 0.8 0.2  
Fair Value 28.4 232.8  
Fixed maturities | Obligations of states and political subdivisions      
Schedule of Invested Assets [Line Items]      
Amortized Cost 109.9 166.7  
Gross Unrealized Gains 0.4 5.8  
Gross Unrealized Losses 10.1 1.2  
Allowance for Credit Losses 0.4 0.0  
Fair Value 99.8 171.3  
Fixed maturities | Corporate bonds      
Schedule of Invested Assets [Line Items]      
Amortized Cost 1,394.8 1,972.3  
Gross Unrealized Gains 0.9 33.5  
Gross Unrealized Losses 160.0 20.3  
Allowance for Credit Losses 1.6 2.2  
Fair Value 1,234.1 1,983.3  
Fixed maturities | Commercial mortgage-backed securities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 337.4 416.7  
Gross Unrealized Gains 0.0 6.3  
Gross Unrealized Losses 52.0 4.3  
Allowance for Credit Losses 0.0 0.0  
Fair Value 285.4 418.7  
Fixed maturities | Residential mortgage-backed securities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 320.0 480.7  
Gross Unrealized Gains 0.2 7.5  
Gross Unrealized Losses 50.2 5.7  
Allowance for Credit Losses 0.0 0.0  
Fair Value 270.0 482.5  
Fixed maturities | Asset-backed securities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 153.4 173.0  
Gross Unrealized Gains 0.0 1.3  
Gross Unrealized Losses 14.2 0.6  
Allowance for Credit Losses 0.0 0.1  
Fair Value 139.2 173.6  
Fixed maturities | Collateralized loan obligations      
Schedule of Invested Assets [Line Items]      
Amortized Cost 254.4 336.4  
Gross Unrealized Gains 0.3 1.3  
Gross Unrealized Losses 16.8 1.6  
Allowance for Credit Losses 0.0 0.0  
Fair Value $ 237.9 $ 336.1