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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Fixed maturities    
Amortized Cost $ 3,716.6 $ 3,529.1
Fair Value 3,745.6 3,460.4 [1]
Fixed Maturities [Member]    
Fixed maturities    
Amortized Cost 3,716.6 3,529.1
Gross Unrealized Gains 74.6 17.5
Gross Unrealized Losses 45.6 86.2
Fair Value 3,745.6 3,460.4
Fixed Maturities [Member] | U.S. Governments [Member]    
Fixed maturities    
Amortized Cost 324.9 240.9
Gross Unrealized Gains 3.9 0.2
Gross Unrealized Losses 0.5 4.9
Fair Value 328.3 236.2
Fixed Maturities [Member] | Foreign Governments [Member]    
Fixed maturities    
Amortized Cost 262.6 224.1
Gross Unrealized Gains 4.2 0.5
Gross Unrealized Losses 2.3 7.8
Fair Value 264.5 216.8
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Fixed maturities    
Amortized Cost 152.6 236.7
Gross Unrealized Gains 7.6 4.3
Gross Unrealized Losses 0.1 1.2
Fair Value 160.1 239.8
Fixed Maturities [Member] | Corporate Bonds [Member]    
Fixed maturities    
Amortized Cost 1,862.0 1,808.7
Gross Unrealized Gains 39.0 7.5
Gross Unrealized Losses 39.2 58.7
Fair Value 1,861.8 1,757.5
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]    
Fixed maturities    
Amortized Cost 213.3 205.3
Gross Unrealized Gains 6.0 0.7
Gross Unrealized Losses 0.3 3.2
Fair Value 219.0 202.8
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]    
Fixed maturities    
Amortized Cost 510.3 413.1
Gross Unrealized Gains 11.9 3.4
Gross Unrealized Losses 0.9 5.7
Fair Value 521.3 410.8
Fixed Maturities [Member] | Asset-backed Securities [Member]    
Fixed maturities    
Amortized Cost 165.6 173.6
Gross Unrealized Gains 1.7 0.4
Gross Unrealized Losses 0.3 1.2
Fair Value 167.0 172.8
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]    
Fixed maturities    
Amortized Cost 225.3 226.7
Gross Unrealized Gains 0.3 0.5
Gross Unrealized Losses 2.0 3.5
Fair Value $ 223.6 $ 223.7
[1] Derived from audited consolidated financial statements.