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
Purchased
Loan portfolio by risk-rating grades
Total	2,031,766	1,489,137
Purchased | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,284,431	901,252
Purchased | Pass
Loan portfolio by risk-rating grades
Total	1,266,429	877,440
Purchased | Watch
Loan portfolio by risk-rating grades
Total	15,326	21,120
Purchased | Substandard
Loan portfolio by risk-rating grades
Total	2,676	2,692
Purchased | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	248,993	106,638
Purchased | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	246,169	102,777
Purchased | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	2,226	2,370
Purchased | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	598	1,491
Purchased | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	81,220	63,846
Purchased | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	81,220	61,206
Purchased | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	0	2,640
Purchased | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	0	0
Purchased | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	100,039	113,294
Purchased | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	93,867	105,265
Purchased | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	5,924	7,665
Purchased | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	248	364
Purchased | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	853,498	617,360
Purchased | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	844,495	608,192
Purchased | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	7,176	8,445
Purchased | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	1,827	723
Purchased | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	681	114
Purchased | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	678	0
Purchased | Installment loans to individuals | Watch
Loan portfolio by risk-rating grades
Total	0	0
Purchased | Installment loans to individuals | Substandard
Loan portfolio by risk-rating grades
Total	$ 3	$ 114