Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Loan portfolio by risk-rating grades
Total	$ 9,088,293	$ 9,083,129
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,308,457	1,295,912
Real estate – construction
Loan portfolio by risk-rating grades
Total	741,073	740,668
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,769,173	2,795,343
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	4,083,632	4,051,509
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	185,958	199,697
Purchased loans
Loan portfolio by risk-rating grades
Total	2,522,694	2,693,417
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,729,943	1,852,090
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,600,416	1,746,683
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	103,302	84,250
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	26,225	21,157
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	387,376	420,263
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	343,031	369,748
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	304,994	333,147
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	31,682	33,857
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	6,355	2,744
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	89,954	105,149
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	85,670	101,964
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	85,670	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	654,265	707,453
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	112,211	128,806
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	99,772	113,874
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	5,741	7,347
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	6,698	7,585
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,357,446	1,423,144
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,189,030	1,251,570
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	1,109,980	1,198,540
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	65,879	43,046
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	13,171	9,984
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	33,653	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