Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,457,308	$ 4,283,833	$ 4,204,704
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,052,109	1,021,056
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	120,785	93,289
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,226,344	1,262,718
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,405,256	1,296,472
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 652,814	$ 610,298