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

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Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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No definition available.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months.

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Amount of deferred tax liability attributable to taxable temporary differences from investments (excludes investments in subsidiaries and equity method investments).

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Amount of realized and unrealized gain (loss) on investment.

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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 of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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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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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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Investment Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
Investment Securities - Additional Information (Detail)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Number of available-for-sale investment securities in the less than twelve month category (security) \| Security	28		28
Number of available-for-sale investment securities in the twelve month or more category (security) \| Security	0		0
Impairment loss on investment securities	$ 0	$ (8,349,000)	$ 0	$ (12,934,000)
Investment securities, at fair value					$ 471,371,000
Unrealized gains (losses) on equity securities	12,602,000	(5,440,000)	(21,651,000)	4,063,000
Deferred tax liability on investment holding gain					0
Gain (Loss) on Investments			(1,533,000)	0
Pledged investment securities fair value	187,100,000		187,100,000		16,900,000
Equity securities
Debt Securities, Available-for-sale [Line Items]
Impairment loss on investment securities		$ (8,300,000)		$ (12,900,000)
Investment securities, at fair value					3,352,000
Unrealized gains (losses) on equity securities					$ 1,000,000
Accounting Standards Update 2016-01
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of new accounting principle in period of adoption						$ 0
Accumulated Other Comprehensive (Loss) Income \| Accounting Standards Update 2016-01
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of new accounting principle in period of adoption						$ 1,000,000
Other Non-interest Income \| Equity securities
Debt Securities, Available-for-sale [Line Items]
Gain (Loss) on Investments	$ 1,200,000		$ (1,500,000)