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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Invested Assets [Line Items]    
Amortized Cost $ 3,320.6 $ 2,938.8
Fair Value 3,343.4 2,932.4
Amortized Cost 338.2 335.2
Fair Value 487.4 447.4
Invested Assets Amortized Cost 4,561.4 4,207.1
Investments Gross Unrealized Gains 213.7 167.3
Investments Gross Unrealized losses 32.2 54.1
Total investments 4,742.9 4,320.3
Fixed Maturities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 3,320.6 2,938.8
Gross Unrealized Gains 50.1 41.9
Gross Unrealized Losses 27.3 48.3
Fair Value 3,343.4 2,932.4
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 222.6 269.6
Gross Unrealized Gains 5.9 3.8
Gross Unrealized Losses 0.5 9.1
Fair Value 228.0 264.3
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 309.3 174.8
Gross Unrealized Gains 2.8 3.7
Gross Unrealized Losses 2.7 1.7
Fair Value 309.4 176.8
Fixed Maturities [Member] | U.S. Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 419.9 275.1
Gross Unrealized Gains 0.2 0.6
Gross Unrealized Losses 5.0 4.5
Fair Value 415.1 271.2
Fixed Maturities [Member] | Foreign Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 229.0 244.2
Gross Unrealized Gains 6.7 1.1
Gross Unrealized Losses 2.5 8.0
Fair Value 233.2 237.3
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 327.7 375.7
Gross Unrealized Gains 9.3 8.9
Gross Unrealized Losses 1.1 1.8
Fair Value 335.9 382.8
Fixed Maturities [Member] | Corporate Bonds [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 1,514.5 1,316.9
Gross Unrealized Gains 24.4 23.3
Gross Unrealized Losses 13.2 19.5
Fair Value 1,525.7 1,320.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 136.3 154.9
Gross Unrealized Gains 0.1 0.4
Gross Unrealized Losses 1.5 1.6
Fair Value 134.9 153.7
Fixed Maturities [Member] | Asset-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 161.3 127.6
Gross Unrealized Gains 0.7 0.1
Gross Unrealized Losses 0.8 2.1
Fair Value 161.2 125.6
Equity Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 338.2 335.2
Gross Unrealized Gains 154.0 117.9
Gross Unrealized Losses 4.8 5.7
Fair Value 487.4 447.4
Other Investments [Member]    
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost 534.1 527.6
Investments Gross Unrealized Gains 9.6 7.5
Investments Gross Unrealized losses 0.1 0.1
Total investments 543.6 535.0
Short-Term Investments [Member]    
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost 368.5 405.5
Investments Gross Unrealized Gains 0.0 0.0
Investments Gross Unrealized losses 0.0 0.0
Total investments $ 368.5 $ 405.5