Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Mar. 31, 2023 USD ($)
Amortized Cost
Due in one year or less	$ 84.1
Due after one year through five years	1,276.3
Due after five years through ten years	509.4
Due after ten years	48.0
Structured securities	1,052.0
Total	2,969.8
Fair Value
Due in one year or less	83.0
Due after one year through five years	1,179.1
Due after five years through ten years	435.5
Due after ten years	38.9
Structured securities	927.9
Total	$ 2,664.4