Investments - Hybrid Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Hybrid Instruments [Line Items]
Hybrid securities	$ 957.6	$ 1,010.3	$ 1,197.3
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	507.1	531.3	548.6
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	300.0	323.9	482.1
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	9.5	13.9	33.8
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 141.0	$ 141.2	$ 132.8