Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Sep. 30, 2022 USD ($)
Amortized Cost
Due in one year or less	$ 174.0
Due after one year through five years	1,921.3
Due after five years through ten years	647.5
Due after ten years	64.7
Structured securities	1,324.0
Total	4,131.5
Fair Value
Due in one year or less	170.8
Due after one year through five years	1,783.4
Due after five years through ten years	542.1
Due after ten years	51.6
Structured securities	1,182.4
Total	$ 3,730.3