Investments - Hybrid Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Hybrid Instruments [Line Items]
Hybrid securities	$ 1,316.7	$ 1,235.3	$ 658.9
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	513.8	479.1	296.7
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	604.3	536.2	163.1
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	62.3	89.2	66.9
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 136.3	$ 130.8	$ 132.2