Securities - Additional Information (Detail) - USD ($)	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 41,272,000	$ 271,824,000
Gross realized gains from securities	370,000	1,700,000
Gross realized losses from securities	3,000	0
Sales of securities held to maturity	0	0
Unrealized gain (loss) on trading securities	36,000	$ (67,000)
Securities sold not yet purchased	16,800,000		$ 4,100,000
Other securities - marketable [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of securities including alternative investment securities	5,000,000		4,600,000
Other Non-marketable Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of securities including alternative investment securities	4,800,000		3,400,000
Collateral Pledged \| U.S. Government and Other Public Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	5,000,000,000		5,700,000,000
Collateral Pledged \| Federal Reserve Discount Window [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	$ 1,600,000,000		$ 1,800,000,000

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
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Amount of cash inflow from the sale or maturity of long-term held-to-maturity securities.

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-Publisher FASB
-Name Accounting Standards Codification
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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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Fair value of financial instruments sold short by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

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Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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