Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Amortized Cost
Due in one year or less	$ 251.2
Due after one year through five years	1,522.2
Due after five years through ten years	586.4
Thereafter	151.9
Structured securities	984.8
Total	3,496.5
Fair Value
Due in one year or less	250.8
Due after one year through five years	1,508.3
Due after five years through ten years	575.7
Thereafter	151.1
Structured securities	970.3
Total	$ 3,456.2