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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, 2020
Dec. 31, 2019
Schedule of Invested Assets [Line Items]    
Amortized Cost $ 3,981.1 $ 3,601.5
Allowance for Credit Losses 6.6 0.0
Fair Value 4,107.1 3,629.9
Fixed maturities    
Schedule of Invested Assets [Line Items]    
Amortized Cost 3,981.1 3,601.5
Gross Unrealized Gains 149.7 68.4
Gross Unrealized Losses 17.1 40.0
Allowance for Credit Losses 6.6 0.0
Fair Value 4,107.1 3,629.9
Fixed maturities | U.S. Governments    
Schedule of Invested Assets [Line Items]    
Amortized Cost 385.4 352.2
Gross Unrealized Gains 14.7 2.3
Gross Unrealized Losses 0.3 1.2
Allowance for Credit Losses 0.0 0.0
Fair Value 399.8 353.3
Fixed maturities | Foreign Governments    
Schedule of Invested Assets [Line Items]    
Amortized Cost 284.1 244.8
Gross Unrealized Gains 11.6 4.6
Gross Unrealized Losses 0.7 0.7
Allowance for Credit Losses 0.2 0.0
Fair Value 294.8 248.7
Fixed maturities | Obligations of states and political subdivisions    
Schedule of Invested Assets [Line Items]    
Amortized Cost 163.1 145.8
Gross Unrealized Gains 7.7 6.9
Gross Unrealized Losses 0.3 0.1
Allowance for Credit Losses 0.1 0.0
Fair Value 170.4 152.6
Fixed maturities | Corporate bonds    
Schedule of Invested Assets [Line Items]    
Amortized Cost 1,925.9 1,775.4
Gross Unrealized Gains 75.3 37.7
Gross Unrealized Losses 13.3 34.7
Allowance for Credit Losses 6.1 0.0
Fair Value 1,981.8 1,778.4
Fixed maturities | Commercial mortgage-backed securities    
Schedule of Invested Assets [Line Items]    
Amortized Cost 324.8 213.5
Gross Unrealized Gains 15.2 4.6
Gross Unrealized Losses 0.3 1.1
Allowance for Credit Losses 0.0 0.0
Fair Value 339.7 217.0
Fixed maturities | Residential mortgage-backed securities    
Schedule of Invested Assets [Line Items]    
Amortized Cost 491.4 479.0
Gross Unrealized Gains 17.4 10.4
Gross Unrealized Losses 0.6 0.6
Allowance for Credit Losses 0.0 0.0
Fair Value 508.2 488.8
Fixed maturities | Asset backed securities    
Schedule of Invested Assets [Line Items]    
Amortized Cost 120.5 164.1
Gross Unrealized Gains 2.9 1.4
Gross Unrealized Losses 0.4 0.2
Allowance for Credit Losses 0.2 0.0
Fair Value 122.8 165.3
Fixed maturities | Collateralized loan obligations    
Schedule of Invested Assets [Line Items]    
Amortized Cost 285.9 226.7
Gross Unrealized Gains 4.9 0.5
Gross Unrealized Losses 1.2 1.4
Allowance for Credit Losses 0.0 0.0
Fair Value $ 289.6 $ 225.8