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Investments Hybrid Securities (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Hybrid Instruments [Line Items]      
Hybrid securities $ 242.2 $ 189.4 $ 188.5
State and local government obligations      
Hybrid Instruments [Line Items]      
Hybrid securities 0.0 3.5 3.5
Corporate debt securities      
Hybrid Instruments [Line Items]      
Hybrid securities 101.0 91.2 91.3
Other asset-backed securities      
Hybrid Instruments [Line Items]      
Hybrid securities 18.7 2.6 3.0
Redeemable preferred stocks      
Hybrid Instruments [Line Items]      
Hybrid securities $ 122.5 $ 92.1 $ 90.7