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Securities - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 295,546,000	$ 257,151,000
Accrued interest on securities available for sale	35,800,000		$ 42,600,000
Sales of securities held to maturity	0	0
Accrued interest on securities held to maturity	6,300,000		5,300,000
Unrealized gain (loss) on trading securities	(154,000)	$ 25,000
Securities sold not yet purchased	2,300,000		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	8,000,000,000.0		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	$ 181,800,000		$ 371,500,000

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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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