Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 6,740,535	$ 6,761,183	$ 6,922,075
Default Status Of TDRs After Number Of Days Past Due	90 days
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 1,374,177	1,555,986	1,773,099
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,360,065	1,541,382
Commercial, financial, agricultural and other [Member] | Commercial Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	14,112	14,604
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	414,816	427,221	416,329
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,828,783	1,750,592	1,723,288
Residential real estate [Member] | Senior Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,218,300	1,144,323
Residential real estate [Member] | Junior Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	610,483	606,269
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,205,758	2,211,569	2,224,710
Commercial Real Estate [Member] | Multifamily
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	385,905	371,239
Commercial Real Estate [Member] | Nonowner Occupied Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,429,192	1,421,151
Commercial Real Estate [Member] | Owner Occupied Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	390,661	419,179
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	917,001	815,815	$ 784,649
Loans to individuals [Member] | Automobiles
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	829,150	712,800
Loans to individuals [Member] | Consumer Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	10,834	12,360
Loans to individuals [Member] | Other Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 77,017	$ 90,655