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Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Schedule of Invested Assets [Line Items]    
Amortized Cost $ 3,248.6 $ 2,938.8 [1]
Fair Value 3,276.2 2,932.4 [1]
Amortized Cost 321.5 335.2 [1]
Fair Value 442.6 447.4 [1]
Invested Assets Amortized Cost 4,466.0 4,211.1
Investments Gross Unrealized Gains 188.7 167.3
Investments Gross Unrealized losses 30.6 54.1
Total investments 4,624.1 4,324.3 [1]
Fixed Maturities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 3,248.6 2,938.8
Gross Unrealized Gains 52.9 41.9
Gross Unrealized Losses 25.3 48.3
Fair Value 3,276.2 2,932.4
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 255.9 269.6
Gross Unrealized Gains 6.1 3.8
Gross Unrealized Losses 1.8 9.1
Fair Value 260.2 264.3
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 221.6 174.8
Gross Unrealized Gains 3.4 3.7
Gross Unrealized Losses 1.7 1.7
Fair Value 223.3 176.8
Fixed Maturities [Member] | U.S. Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 317.7 275.1
Gross Unrealized Gains 0.6 0.6
Gross Unrealized Losses 3.2 4.5
Fair Value 315.1 271.2
Fixed Maturities [Member] | Foreign Governments [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 244.5 244.2
Gross Unrealized Gains 3.3 1.1
Gross Unrealized Losses 3.6 8.0
Fair Value 244.2 237.3
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 337.3 375.7
Gross Unrealized Gains 11.9 8.9
Gross Unrealized Losses 0.8 1.8
Fair Value 348.4 382.8
Fixed Maturities [Member] | Corporate Bonds [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 1,596.9 1,316.9
Gross Unrealized Gains 26.9 23.3
Gross Unrealized Losses 11.9 19.5
Fair Value 1,611.9 1,320.7
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 145.0 154.9
Gross Unrealized Gains 0.3 0.4
Gross Unrealized Losses 1.3 1.6
Fair Value 144.0 153.7
Fixed Maturities [Member] | Asset-backed Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 129.7 127.6
Gross Unrealized Gains 0.4 0.1
Gross Unrealized Losses 1.0 2.1
Fair Value 129.1 125.6
Equity Securities [Member]    
Schedule of Invested Assets [Line Items]    
Amortized Cost 321.5 335.2
Gross Unrealized Gains 126.3 117.9
Gross Unrealized Losses 5.2 5.7
Fair Value 442.6 447.4
Other Investments [Member]    
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost 530.0 531.6
Investments Gross Unrealized Gains 9.5 7.5
Investments Gross Unrealized losses 0.1 0.1
Total investments 539.4 539.0
Short-Term Investments [Member]    
Schedule of Invested Assets [Line Items]    
Invested Assets Amortized Cost 365.9 405.5
Investments Gross Unrealized Gains 0.0 0.0
Investments Gross Unrealized losses 0.0 0.0
Total investments $ 365.9 $ 405.5
[1] Derived from audited consolidated financial statements.