Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Loan portfolio by risk-rating grades
Total	$ 9,313,784	$ 9,083,129
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,328,560	1,295,912
Real estate – construction
Loan portfolio by risk-rating grades
Total	816,695	740,668
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,797,633	2,795,343
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	4,022,375	4,051,509
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	348,521	199,697
Purchased loans
Loan portfolio by risk-rating grades
Total	2,281,966	2,693,417
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,501,133	1,852,090
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,418,595	1,746,683
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	55,911	84,250
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	26,627	21,157
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	339,693	420,263
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	294,277	369,748
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	281,746	333,147
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	7,323	33,857
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	5,208	2,744
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	52,106	105,149
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	49,431	101,964
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	49,431	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	561,725	707,453
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	90,036	128,806
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	80,714	113,874
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	3,874	7,347
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	5,448	7,585
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,212,905	1,423,144
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,067,389	1,251,570
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	1,006,704	1,198,540
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	44,714	43,046
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	15,971	9,984
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	115,537	37,408
Purchased loans | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	0	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	$ 0	$ 2