Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Sep. 30, 2021 USD ($)
Amortized Cost
Due in one year or less	$ 288.7
Due after one year through five years	1,670.6
Due after five years through ten years	719.6
Due after ten years	121.3
Structured securities	1,345.0
Total	4,145.2
Fair Value
Due in one year or less	292.9
Due after one year through five years	1,695.1
Due after five years through ten years	728.6
Due after ten years	121.4
Structured securities	1,359.3
Total	$ 4,197.3