Investment Securities - Narrative (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale investment securities in the less than twelve month category (security) \| Security	20
Number of available-for-sale investment securities in the twelve month or more category (security) \| Security	16
Amortized cost	$ 472,300	$ 497,869
Investment securities available for sale, fair value	471,371	493,474
Unrealized gains (losses) on available-for-sale securities	2,520	2,505	$ (6,328)
Deferred tax liabilities, equity investments	0
Pledged investment securities fair value	16,900	231,300
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Impairment loss	12,900
Amortized cost	2,311	15,246		$ 2,300
Investment securities available for sale, fair value	3,352	$ 15,246
Unrealized gains (losses) on available-for-sale securities	$ 1,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ References

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from equity method investments.

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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Sum of the carrying amounts as of the date of the latest financial statement presented of securities which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

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