Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Jun. 30, 2019 USD ($)
Amortized Cost
Due in one year or less	$ 268.1
Due after one year through five years	1,439.4
Due after five years through ten years	622.8
Thereafter	86.9
Structured securities	1,082.3
Total	3,499.5
Fair Value
Due in one year or less	268.2
Due after one year through five years	1,444.4
Due after five years through ten years	634.5
Thereafter	91.2
Structured securities	1,094.9
Total	$ 3,533.2