Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Jun. 30, 2017 USD ($)
Investments Debt And Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 256.5
Amortized Cost, Due after one year through five years	1,463.5
Amortized Cost, Due after five years through ten years	614.2
Amortized Cost, Thereafter	162.2
Amortized Cost, Structured securities	752.2
Amortized Cost, Total	3,248.6
Fair Value, Due in one year or less	253.4
Fair Value, Due after one year through five years	1,475.8
Fair Value, Due after five years through ten years	623.6
Fair Value, Thereafter	166.8
Fair Value, Structured securities	756.6
Fair Value, Total	$ 3,276.2