Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 6,761,183	$ 6,189,148	$ 5,774,139
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,555,986	1,241,853
Commercial, financial, agricultural and other [Member] | Time and demand loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,541,382
Commercial, financial, agricultural and other [Member] | Commercial credit cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	14,604
Commercial, financial, agricultural and other [Member] | Senior Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,144,323
Commercial, financial, agricultural and other [Member] | Junior Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	606,269
Commercial, financial, agricultural and other [Member] | Multifamily
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	371,239
Commercial, financial, agricultural and other [Member] | Nonowner Occupied Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,421,151
Commercial, financial, agricultural and other [Member] | Owner Occupied Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	419,179
Commercial, financial, agricultural and other [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	712,800
Commercial, financial, agricultural and other [Member] | Consumer Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	12,360
Commercial, financial, agricultural and other [Member] | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	90,655
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	427,221	449,039
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,750,592	1,681,362
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,211,569	2,117,519
Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 815,815	699,375
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		5,714,373
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		1,212,026
Originated Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		442,777
Originated Loans [Member] | Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		1,415,808
Originated Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		1,958,346
Originated Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		685,416
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		474,775
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		29,827
Acquired Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		6,262
Acquired Loans [Member] | Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		265,554
Acquired Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		159,173
Acquired Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans		$ 13,959