Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Amortized Cost
Due in one year or less	$ 78.0
Due after one year through five years	1,268.4
Due after five years through ten years	556.3
Due after ten years	48.5
Total	3,016.4	$ 4,203.2
Fair Value
Due in one year or less	76.6
Due after one year through five years	1,162.6
Due after five years through ten years	465.1
Due after ten years	38.7
Total	2,675.5	$ 4,223.3
Structured Securities
Amortized Cost
Structured securities	1,065.2
Fair Value
Structured securities	$ 932.5