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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, 2018
Dec. 31, 2017
Schedule of Invested Assets [Line Items]    
Amortized Cost $ 3,529.1 $ 3,320.6
Fair Value 3,460.4 3,343.4
Fixed Maturities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 3,529.1 3,320.6
Gross Unrealized Gains 17.5 50.1
Gross Unrealized Losses 86.2 27.3
Fair Value 3,460.4 3,343.4
Fixed Maturities [Member] | U.S. Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 240.9 419.9
Gross Unrealized Gains 0.2 0.2
Gross Unrealized Losses 4.9 5.0
Fair Value 236.2 415.1
Fixed Maturities [Member] | Foreign Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 224.1 229.0
Gross Unrealized Gains 0.5 6.7
Gross Unrealized Losses 7.8 2.5
Fair Value 216.8 233.2
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 236.7 327.7
Gross Unrealized Gains 4.3 9.3
Gross Unrealized Losses 1.2 1.1
Fair Value 239.8 335.9
Fixed Maturities [Member] | Corporate Bonds [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 1,808.7 1,514.5
Gross Unrealized Gains 7.5 24.4
Gross Unrealized Losses 58.7 13.2
Fair Value 1,757.5 1,525.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 205.3 136.3
Gross Unrealized Gains 0.7 0.1
Gross Unrealized Losses 3.2 1.5
Fair Value 202.8 134.9
Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 413.1 309.3
Gross Unrealized Gains 3.4 2.8
Gross Unrealized Losses 5.7 2.7
Fair Value 410.8 309.4
Fixed Maturities [Member] | Asset-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 173.6 161.3
Gross Unrealized Gains 0.4 0.7
Gross Unrealized Losses 1.2 0.8
Fair Value 172.8 161.2
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 226.7 222.6
Gross Unrealized Gains 0.5 5.9
Gross Unrealized Losses 3.5 0.5
Fair Value $ 223.7 $ 228.0