Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Hybrid Instruments [Line Items]
Hybrid securities	$ 188.5	$ 244.7	$ 240.3
State and local government obligations
Hybrid Instruments [Line Items]
Hybrid securities	3.5	3.6	3.6
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	91.3	158.9	159.7
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	3.0	4.5	5.0
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 90.7	$ 77.7	$ 72.0