Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Loan portfolio by risk-rating grades
Total	$ 7,448,608	$ 6,202,709
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,008,935	717,490
Real estate – construction
Loan portfolio by risk-rating grades
Total	577,720	552,679
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,295,852	1,878,177
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	3,390,389	2,898,895
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	175,712	155,468
Purchased loans
Loan portfolio by risk-rating grades
Total	2,155,141	1,489,137
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,364,573	901,252
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,345,814	877,440
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	16,006	21,120
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	2,753	2,692
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	301,100	128,200
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	272,842	106,638
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	269,798	102,777
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	2,381	2,370
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	663	1,491
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	100,082	68,753
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	92,825	63,846
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	92,825	61,206
Purchased loans | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	0	2,640
Purchased loans | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	0	0
Purchased loans | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	651,792	452,447
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	111,334	113,294
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	105,040	105,265
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	6,042	7,665
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	252	364
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,079,049	823,758
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	886,873	617,360
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	877,455	608,192
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	7,583	8,445
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	1,835	723
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	23,118	15,979
Purchased loans | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	699	114
Purchased loans | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	696	0
Purchased loans | Installment loans to individuals | Watch
Loan portfolio by risk-rating grades
Total	0	0
Purchased loans | Installment loans to individuals | Substandard
Loan portfolio by risk-rating grades
Total	$ 3	$ 114