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Investment Securities - Additional Information (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale investment securities in the less than twelve month category (security) | Security	62
Number of available-for-sale investment securities in the twelve month or more category (security) | Security	16
Impairment loss on investment securities	$ 0	$ (1,703)
Investment securities, at fair value	1,181,661		$ 471,371
Unrealized gains (losses) on equity securities	(25,233)	(685)
Gain (Loss) on Investments	10	0
Pledged investment securities fair value	701,500		16,900
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Impairment loss on investment securities		$ (1,700)
Investment securities, at fair value			3,352
Unrealized gains (losses) on equity securities			1,000
Accounting Standards Update 2016-01
Schedule of Available-for-sale Securities [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ 0
Accumulated Other Comprehensive Loss | Accounting Standards Update 2016-01
Schedule of Available-for-sale Securities [Line Items]
Cumulative effect of new accounting principle in period of adoption				$ 1,000
Other Non-interest Income | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gain (Loss) on Investments	$ 10

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- Definition

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

+ 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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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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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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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject 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 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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