Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Hybrid Instruments [Line Items]
Hybrid securities	$ 818.0	$ 354.6	$ 242.2
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	363.3	188.4	101.0
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	217.0	0.0	0.0
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	104.5	34.8	18.7
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 133.2	$ 131.4	$ 122.5