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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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