Non Purchased Loans - Loan Portfolio by Risk-rating Grades (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loan portfolio by risk-rating grades
Total	$ 7,620,322	$ 6,202,709
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,039,393	717,490
Real estate – construction
Loan portfolio by risk-rating grades
Total	633,389	552,679
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,343,721	1,878,177
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	3,427,530	2,898,895
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	176,289	155,468
Non-Purchased
Loan portfolio by risk-rating grades
Total	5,588,556	4,713,572
Non-Purchased | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	3,378,267	2,808,367
Non-Purchased | Pass
Loan portfolio by risk-rating grades
Total	3,340,846	2,761,884
Non-Purchased | Watch
Loan portfolio by risk-rating grades
Total	22,504	26,416
Non-Purchased | Substandard
Loan portfolio by risk-rating grades
Total	14,917	20,067
Non-Purchased | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	570,841	439,704
Non-Purchased | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	566,439	434,323
Non-Purchased | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	3,913	4,531
Non-Purchased | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	489	850
Non-Purchased | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	484,241	402,549
Non-Purchased | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	484,160	402,156
Non-Purchased | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	81	393
Non-Purchased | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	0	0
Non-Purchased | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	263,845	200,385
Non-Purchased | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	255,148	190,882
Non-Purchased | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	4,977	3,374
Non-Purchased | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	3,720	6,129
Non-Purchased | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	2,058,419	1,765,729
Non-Purchased | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	2,034,178	1,734,523
Non-Purchased | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	13,533	18,118
Non-Purchased | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	10,708	13,088
Non-Purchased | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	921	0
Non-Purchased | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	921	0
Non-Purchased | Installment loans to individuals | Watch
Loan portfolio by risk-rating grades
Total	0	0
Non-Purchased | Installment loans to individuals | Substandard
Loan portfolio by risk-rating grades
Total	$ 0	$ 0