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Summary of Portfolio of Securities Held to Maturity (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Held-to-maturity Securities
Schedule of Held-to-maturity Securities [Line Items]
Non-credit portion of OTTI recorded in AOCL, pre-tax	$ 8.6	$ 8.7

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- Definition

Accumulated Other Comprehensive Income Loss Noncredit Portion of Other Than Temporarily Impaired Securities Before Tax

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No definition available.

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Period Type:	instant

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- Definition

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.

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No definition available.

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