Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Sep. 30, 2020 USD ($)
Amortized Cost
Due in one year or less	$ 303.4
Due after one year through five years	1,682.0
Due after five years through ten years	603.4
Thereafter	65.8
Structured securities	1,192.4
Total	3,847.0
Fair Value
Due in one year or less	305.4
Due after one year through five years	1,691.7
Due after five years through ten years	622.9
Thereafter	67.4
Structured securities	1,227.0
Total	$ 3,914.4