Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 4,437,601	$ 4,457,308	$ 4,252,213
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,066,788	1,052,109	1,017,412
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	107,882	120,785	95,110
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,210,511	1,226,344	1,240,169
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,400,276	1,405,256	1,290,852
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 652,144	$ 652,814	$ 608,670