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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

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