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Investments - Cost and Fair Value of Available-for-Sale Fixed-Maturity Securities by Contractual Maturity (Detail) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Amortized Cost and Fair Value Debt Securities [Abstract]    
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Amortized Cost $ 343.2  
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Amortized Cost 123.2  
Available-for-sale Securities, Amortized Cost 466.4 $ 869.3
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Fair Value 343.3  
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Fair Value 123.2  
Available-for-sale Securities, Fair Value $ 466.5 $ 868.9