Loans and Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
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	$ 25,900		$ 12,600
Other real estate owned	25,000		21,000
Residential real estate loans secured by residential real estate properties for which formal foreclosure proceedings were in process	49,000		48,000
Interest Rate Concession
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
TDRs	465	$ 1,900
Modification of Loan Structure
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
TDRs	84,300	$ 7,000
Energy Portfolio Segment
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	7,900,000		8,100,000
Commercial, financial and agricultural	2,900,000		3,200,000
Residential real estate – mortgage
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Other real estate owned	$ 22,000		$ 18,000