Investments - Hybrid Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Hybrid Instruments [Line Items]
Hybrid securities	$ 888.3	$ 1,010.3	$ 1,120.3
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	475.5	531.3	520.1
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	272.5	323.9	448.7
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	5.3	13.9	26.1
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 135.0	$ 141.2	$ 125.4