Securities - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 129,128,000	$ 84,439,000
Gross realized gains from securities	2,720,000	1,227,000
Gross realized losses from securities	0	0
Accrued interest on securities available for sale	33,600,000		$ 42,600,000
Sales of securities held to maturity	0	0
Accrued interest on securities held to maturity	6,700,000		5,300,000
Unrealized gain (loss) on trading securities	(40,000)	150,000
Securities sold not yet purchased		$ 2,000,000.0	2,200,000
Collateral Dependent Assets [Member] \| U.S. Government and Other Public Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	7,300,000,000		7,800,000,000
Collateral Dependent Assets [Member] \| Federal Reserve Discount Window [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	$ 214,900,000		$ 371,500,000

Accrued interest receivable on debt securities available for sale.

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Accrued interest receivable on debt securities held to maturity.

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 9
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Amount of realized loss on 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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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
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-Section 50
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Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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Amount of cash inflow from the sale or maturity of long-term held-to-maturity securities.

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Amount of cash inflow from sale of 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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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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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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