Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ (6,732,580)	$ (6,761,183)	$ (6,949,716)
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(1,224,591)	(1,555,986)	(1,736,736)
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(1,210,694)	(1,541,382)
Commercial, financial, agricultural and other [Member] | Commercial Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(13,897)	(14,604)
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(424,016)	(427,221)	(452,989)
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(1,875,375)	(1,750,592)	(1,744,020)
Residential real estate [Member] | Senior Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(1,254,142)	(1,144,323)
Residential real estate [Member] | Junior Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(621,233)	(606,269)
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(2,231,890)	(2,211,569)	(2,215,311)
Commercial Real Estate [Member] | Multifamily
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(405,720)	(371,239)
Commercial Real Estate [Member] | Nonowner Occupied Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(1,442,845)	(1,421,151)
Commercial Real Estate [Member] | Owner Occupied Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(383,325)	(419,179)
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(976,708)	(815,815)	$ (800,660)
Loans to individuals [Member] | Automobiles
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(886,367)	(712,800)
Loans to individuals [Member] | Consumer Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	(10,617)	(12,360)
Loans to individuals [Member] | Other Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ (79,724)	$ (90,655)