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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, 2021
Dec. 31, 2020
Dec. 31, 2019
Schedule of Invested Assets [Line Items]      
Amortized Cost $ 4,203.2 $ 3,981.1  
Allowance for Credit Losses 2.5 6.6 $ 0.0
Fair Value 4,223.3 4,107.1  
Fixed maturities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 4,203.2 3,981.1  
Gross Unrealized Gains 63.4 149.7  
Gross Unrealized Losses 40.8 17.1  
Allowance for Credit Losses 2.5 6.6  
Fair Value 4,223.3 4,107.1  
Fixed maturities | U.S. Governments      
Schedule of Invested Assets [Line Items]      
Amortized Cost 422.7 385.4  
Gross Unrealized Gains 5.5 14.7  
Gross Unrealized Losses 3.2 0.3  
Allowance for Credit Losses 0.0 0.0  
Fair Value 425.0 399.8  
Fixed maturities | Foreign Governments      
Schedule of Invested Assets [Line Items]      
Amortized Cost 234.7 284.1  
Gross Unrealized Gains 2.2 11.6  
Gross Unrealized Losses 3.9 0.7  
Allowance for Credit Losses 0.2 0.2  
Fair Value 232.8 294.8  
Fixed maturities | Obligations of states and political subdivisions      
Schedule of Invested Assets [Line Items]      
Amortized Cost 166.7 163.1  
Gross Unrealized Gains 5.8 7.7  
Gross Unrealized Losses 1.2 0.3  
Allowance for Credit Losses 0.0 0.1  
Fair Value 171.3 170.4  
Fixed maturities | Corporate bonds      
Schedule of Invested Assets [Line Items]      
Amortized Cost 1,972.3 1,925.9  
Gross Unrealized Gains 33.5 75.3  
Gross Unrealized Losses 20.3 13.3  
Allowance for Credit Losses 2.2 6.1  
Fair Value 1,983.3 1,981.8  
Fixed maturities | Commercial mortgage-backed securities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 416.7 324.8  
Gross Unrealized Gains 6.3 15.2  
Gross Unrealized Losses 4.3 0.3  
Allowance for Credit Losses 0.0 0.0  
Fair Value 418.7 339.7  
Fixed maturities | Residential mortgage-backed securities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 480.7 491.4  
Gross Unrealized Gains 7.5 17.4  
Gross Unrealized Losses 5.7 0.6  
Allowance for Credit Losses 0.0 0.0  
Fair Value 482.5 508.2  
Fixed maturities | Asset backed securities      
Schedule of Invested Assets [Line Items]      
Amortized Cost 173.0 120.5  
Gross Unrealized Gains 1.3 2.9  
Gross Unrealized Losses 0.6 0.4  
Allowance for Credit Losses 0.1 0.2  
Fair Value 173.6 122.8  
Fixed maturities | Collateralized loan obligations      
Schedule of Invested Assets [Line Items]      
Amortized Cost 336.4 285.9  
Gross Unrealized Gains 1.3 4.9  
Gross Unrealized Losses 1.6 1.2  
Allowance for Credit Losses 0.0 0.0  
Fair Value $ 336.1 $ 289.6