Loans and the Allowance for Loan Losses - (Details 6) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Loan portfolio by risk-rating grades
Total	$ 3,146,274	$ 3,066,005
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	475,212	475,417
Lease financing
Loan portfolio by risk-rating grades
Total	0	0
Real estate – construction
Loan portfolio by risk-rating grades
Total	289,848	273,881
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	307,433	300,441
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	2,073,590	2,016,210
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	191	56
Pass
Loan portfolio by risk-rating grades
Total	3,070,144	2,982,255
Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	463,922	465,185
Pass | Lease financing
Loan portfolio by risk-rating grades
Total	0	0
Pass | Real estate – construction
Loan portfolio by risk-rating grades
Total	289,169	273,398
Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	287,219	275,269
Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	2,029,759	1,968,352
Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	75	51
Watch
Loan portfolio by risk-rating grades
Total	39,405	45,868
Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	9,302	8,498
Watch | Lease financing
Loan portfolio by risk-rating grades
Total	0	0
Watch | Real estate – construction
Loan portfolio by risk-rating grades
Total	679	483
Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	8,102	9,712
Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	21,322	27,175
Watch | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	0	0
Substandard
Loan portfolio by risk-rating grades
Total	36,725	37,882
Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,988	1,734
Substandard | Lease financing
Loan portfolio by risk-rating grades
Total	0	0
Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Total	0	0
Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	12,112	15,460
Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	22,509	20,683
Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	$ 116	$ 5