Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Mar. 31, 2024 USD ($)
Amortized Cost
Due in one year or less	$ 347.3
Due after one year through five years	950.2
Due after five years through ten years	231.5
Due after ten years	15.6
Structured securities	825.4
Total	2,370.0
Fair Value
Due in one year or less	347.6
Due after one year through five years	961.3
Due after five years through ten years	240.5
Due after ten years	17.0
Structured securities	847.0
Total	$ 2,413.4