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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fixed maturities    
Amortized Cost $ 3,338.5 $ 3,320.6
Fair Value 3,301.4 3,343.4 [1]
Fixed Maturities [Member]    
Fixed maturities    
Amortized Cost 3,338.5 3,320.6
Gross Unrealized Gains 22.6 50.1
Gross Unrealized Losses 59.7 27.3
Fair Value 3,301.4 3,343.4
Fixed Maturities [Member] | U.S. Governments [Member]    
Fixed maturities    
Amortized Cost 268.0 419.9
Gross Unrealized Gains 0.2 0.2
Gross Unrealized Losses 7.0 5.0
Fair Value 261.2 415.1
Fixed Maturities [Member] | Foreign Governments [Member]    
Fixed maturities    
Amortized Cost 248.3 229.0
Gross Unrealized Gains 1.7 6.7
Gross Unrealized Losses 5.9 2.5
Fair Value 244.1 233.2
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Fixed maturities    
Amortized Cost 266.6 327.7
Gross Unrealized Gains 5.9 9.3
Gross Unrealized Losses 1.8 1.1
Fair Value 270.7 335.9
Fixed Maturities [Member] | Corporate Bonds [Member]    
Fixed maturities    
Amortized Cost 1,655.2 1,514.5
Gross Unrealized Gains 10.7 24.4
Gross Unrealized Losses 30.5 13.2
Fair Value 1,635.4 1,525.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]    
Fixed maturities    
Amortized Cost 145.8 136.3
Gross Unrealized Gains 0.0 0.1
Gross Unrealized Losses 2.8 1.5
Fair Value 143.0 134.9
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]    
Fixed maturities    
Amortized Cost 375.7 309.3
Gross Unrealized Gains 2.0 2.8
Gross Unrealized Losses 9.6 2.7
Fair Value 368.1 309.4
Fixed Maturities [Member] | Asset-backed Securities [Member]    
Fixed maturities    
Amortized Cost 159.4 161.3
Gross Unrealized Gains 0.4 0.7
Gross Unrealized Losses 1.3 0.8
Fair Value 158.5 161.2
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]    
Fixed maturities    
Amortized Cost 219.5 222.6
Gross Unrealized Gains 1.7 5.9
Gross Unrealized Losses 0.8 0.5
Fair Value $ 220.4 $ 228.0
[1] Derived from audited consolidated financial statements.