Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Hybrid Instruments [Line Items]
Hybrid securities	$ 658.9	$ 354.6	$ 180.1
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	296.7	188.4	84.3
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	163.1	0.0	0.0
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	66.9	34.8	2.2
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 132.2	$ 131.4	$ 90.2