Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Detail) $ in Millions	Jun. 30, 2022 USD ($)
Amortized Cost
Due in one year or less	$ 166.4
Due after one year through five years	1,861.8
Due after five years through ten years	688.9
Due after ten years	76.5
Structured securities	1,362.9
Total	4,156.5
Fair Value
Due in one year or less	166.0
Due after one year through five years	1,762.9
Due after five years through ten years	603.1
Due after ten years	65.2
Structured securities	1,251.8
Total	$ 3,849.0