Loans and the Allowance for Loan Losses - (Details 6) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Loan portfolio by risk-rating grades
Total	$ 3,493,580	$ 3,066,005
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	515,405	475,417
Lease financing
Loan portfolio by risk-rating grades
Total	0	0
Real estate – construction
Loan portfolio by risk-rating grades
Total	360,882	273,881
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	320,063	300,441
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	2,297,011	2,016,210
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	219	56
Pass
Loan portfolio by risk-rating grades
Total	3,420,012	2,982,255
Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	506,028	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	359,523	273,398
Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	297,907	275,269
Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	2,256,451	1,968,352
Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	103	51
Watch
Loan portfolio by risk-rating grades
Total	41,534	45,868
Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	7,346	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	1,192	483
Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	9,918	9,712
Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	23,078	27,175
Watch | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	0	0
Substandard
Loan portfolio by risk-rating grades
Total	32,034	37,882
Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	2,031	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	167	0
Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	12,238	15,460
Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	17,482	20,683
Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	$ 116	$ 5