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Investments Hybrid Securities (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Hybrid Securities [Line Items]    
Hybrid securities $ 142.9 $ 219.5
Fixed maturities | State and local government obligations    
Hybrid Securities [Line Items]    
Hybrid securities 6.1 0.0
Fixed maturities | Corporate debt securities    
Hybrid Securities [Line Items]    
Hybrid securities 99.8 40.1
Fixed maturities | Residential mortgage-backed securities    
Hybrid Securities [Line Items]    
Hybrid securities 0.0 170.5
Fixed maturities | Other asset-backed securities    
Hybrid Securities [Line Items]    
Hybrid securities 6.7 8.9
Fixed maturities | Redeemable preferred stocks    
Hybrid Securities [Line Items]    
Hybrid securities $ 30.3 $ 0.0