Securities - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 257,151,000	$ 331,399,000
Gross realized gains from securities	5,700,000	3,900,000
Gross realized losses from securities	171,000	1,400,000
Accrued interest on securities available for sale	32,500,000
ACL related to AFS Securities	0
Sales of securities held to maturity	0	0
Accrued interest on securities held to maturity	6,800,000
Unrealized gains on trading securities	25,000	$ (97,000)
Securities sold not yet purchased	2,500,000		$ 14,600,000
Other Non-marketable Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of securities including alternative investment securities	3,000,000.0		7,000,000.0
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	6,900,000,000		5,800,000,000
Collateral Dependent Assets [Member] \| Federal Reserve Discount Window [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	$ 393,600,000		$ 481,200,000

Accrued interest receivable on debt securities available for sale.

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Accrued interest receivable on debt securities held to maturity.

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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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Amount of allowance for credit 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 realized gain 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 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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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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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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