Investments - Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Hybrid Instruments [Line Items]
Hybrid securities	$ 1,008.9	$ 1,010.3	$ 1,077.7
Corporate and other debt securities
Hybrid Instruments [Line Items]
Hybrid securities	637.3	672.5	647.5
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	369.0	323.9	410.8
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	$ 2.6	$ 13.9	$ 19.4