Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Sep. 30, 2019 USD ($)
Amortized Cost
Due in one year or less	$ 242.5
Due after one year through five years	1,584.3
Due after five years through ten years	669.5
Thereafter	105.8
Structured securities	1,114.5
Total	3,716.6
Fair Value
Due in one year or less	242.2
Due after one year through five years	1,577.1
Due after five years through ten years	683.5
Thereafter	111.9
Structured securities	1,130.9
Total	$ 3,745.6