Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Amortized Cost
Due in one year or less	$ 270.5
Due after one year through five years	1,517.0
Due after five years through ten years	581.5
Thereafter	141.4
Structured securities	1,018.7
Total	3,529.1
Fair Value
Due in one year or less	266.3
Due after one year through five years	1,486.2
Due after five years through ten years	557.1
Thereafter	140.7
Structured securities	1,010.1
Total	$ 3,460.4