Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Mar. 31, 2022 USD ($)
Amortized Cost
Due in one year or less	$ 229.3
Due after one year through five years	1,849.4
Due after five years through ten years	719.4
Due after ten years	88.2
Structured securities	1,416.4
Total	4,302.7
Fair Value
Due in one year or less	232.1
Due after one year through five years	1,795.6
Due after five years through ten years	673.7
Due after ten years	82.5
Structured securities	1,360.0
Total	$ 4,143.9