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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
Mar. 31, 2018
Dec. 31, 2017
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost $ 4,721.1 $ 4,561.4
Investments Gross Unrealized Gains 175.3 213.7
Investments Gross Unrealized losses 62.9 32.2
Total investments 4,833.5 4,742.9 [1]
Amortized Cost 3,306.7 3,320.6 [1]
Fair Value 3,291.5 3,343.4 [1]
Amortized Cost 358.3 338.2 [1]
Fair Value 476.5 487.4 [1]
Fixed Maturities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 3,306.7 3,320.6
Gross Unrealized Gains 33.8 50.1
Gross Unrealized Losses 49.0 27.3
Fair Value 3,291.5 3,343.4
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 192.3 222.6
Gross Unrealized Gains 4.8 5.9
Gross Unrealized Losses 0.3 0.5
Fair Value 196.8 228.0
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 341.5 309.3
Gross Unrealized Gains 2.3 2.8
Gross Unrealized Losses 8.1 2.7
Fair Value 335.7 309.4
Fixed Maturities [Member] | U.S. Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 374.2 419.9
Gross Unrealized Gains 0.2 0.2
Gross Unrealized Losses 7.4 5.0
Fair Value 367.0 415.1
Fixed Maturities [Member] | Foreign Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 233.0 229.0
Gross Unrealized Gains 3.9 6.7
Gross Unrealized Losses 3.5 2.5
Fair Value 233.4 233.2
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 315.9 327.7
Gross Unrealized Gains 6.6 9.3
Gross Unrealized Losses 2.0 1.1
Fair Value 320.5 335.9
Fixed Maturities [Member] | Corporate Bonds [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 1,580.7 1,514.5
Gross Unrealized Gains 15.0 24.4
Gross Unrealized Losses 24.1 13.2
Fair Value 1,571.6 1,525.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 103.5 136.3
Gross Unrealized Gains 0.0 0.1
Gross Unrealized Losses 2.3 1.5
Fair Value 101.2 134.9
Fixed Maturities [Member] | Asset-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 165.6 161.3
Gross Unrealized Gains 1.0 0.7
Gross Unrealized Losses 1.3 0.8
Fair Value 165.3 161.2
Equity Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 358.3 338.2
Gross Unrealized Gains 131.9 154.0
Gross Unrealized Losses 13.7 4.8
Fair Value 476.5 487.4
Other Investments [Member]    
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost 513.7 534.1
Investments Gross Unrealized Gains 9.6 9.6
Investments Gross Unrealized losses 0.0 0.1
Total investments 523.3 543.6
Short-Term Investments [Member]    
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost 542.4 368.5
Investments Gross Unrealized Gains 0.0 0.0
Investments Gross Unrealized losses 0.2 0.0
Total investments $ 542.2 $ 368.5
[1] Derived from audited consolidated financial statements.