Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Loan portfolio by risk-rating grades
Total	$ 9,054,654	$ 9,083,129
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,305,076	1,295,912
Real estate – construction
Loan portfolio by risk-rating grades
Total	781,531	740,668
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,765,472	2,795,343
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	4,017,969	4,051,509
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	184,606	199,697
Purchased loans
Loan portfolio by risk-rating grades
Total	2,350,366	2,693,417
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,628,846	1,852,090
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,503,584	1,746,683
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	92,831	84,250
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	32,431	21,157
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	374,478	420,263
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	336,797	369,748
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	299,893	333,147
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	24,250	33,857
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	12,654	2,744
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	65,402	105,149
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	63,553	101,964
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	63,553	101,122
Purchased loans | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	0	0
Purchased loans | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	0	842
Purchased loans | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	604,855	707,453
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	103,181	128,806
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	91,968	113,874
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	4,979	7,347
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	6,234	7,585
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,276,567	1,423,144
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,125,314	1,251,570
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	1,048,170	1,198,540
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	63,602	43,046
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	13,542	9,984
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	29,064	37,408
Purchased loans | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1	2
Purchased loans | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	0	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	$ 1	$ 2