Summary of Portfolio of Securities Available for Sale (Parenthetical) (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Non-credit portion of OTTI recorded in AOCL, pre-tax	$ 8.6

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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