Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Hybrid Instruments [Line Items]
Hybrid securities	$ 489.4	$ 354.6	$ 190.5
State and local government obligations
Hybrid Instruments [Line Items]
Hybrid securities	0.0	0.0	3.4
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	295.0	188.4	116.5
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	35.0	0.0	0.0
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	31.9	34.8	2.2
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 127.5	$ 131.4	$ 68.4