Loans and Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Commitments to lend additional funds to borrowers whose terms have been modified in a TDR	$ 4,000		$ 5,700
Other real estate owned	17,877		20,862
Residential real estate loans secured by residential real estate properties for which formal foreclosure proceedings were in process	29,000		30,000
Commercial, financial and agricultural	26,864,047		26,022,374
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Other real estate owned	16,000		17,000
Energy Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Total energy exposure, including unused commitments to extend credit and letters of credit	9,300,000		9,400,000
Commercial, financial and agricultural	4,200,000		$ 3,800,000
Interest Rate Concession [Member]
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
TDRs	1,900	$ 300
Modification of Loan Structure [Member]
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
TDRs	$ 7,000	$ 4,500