Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 6,189,148	$ 5,774,139	$ 5,407,376
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,241,853	1,138,473
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	449,039	358,978
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,681,362	1,562,405
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,117,519	2,123,544
Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	699,375	590,739
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5,714,373	5,275,296
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,212,026	1,100,947
Originated Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	442,777	353,008
Originated Loans [Member] | Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,415,808	1,313,645
Originated Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,958,346	1,922,349
Originated Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	685,416	585,347
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	474,775	498,843
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	29,827	37,526
Acquired Loans [Member] | Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,262	5,970
Acquired Loans [Member] | Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	265,554	248,760
Acquired Loans [Member] | Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	159,173	201,195
Acquired Loans [Member] | Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 13,959	$ 5,392