Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loan portfolio by risk-rating grades
Total	$ 7,698,070	$ 7,620,322
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,046,818	1,039,393
Real estate – construction
Loan portfolio by risk-rating grades
Total	657,491	633,389
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,358,101	2,343,721
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	3,463,953	3,427,530
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	171,707	176,289
Purchased loans
Loan portfolio by risk-rating grades
Total	1,867,948	2,031,766
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,168,137	1,284,431
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,120,830	1,241,834
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	24,967	24,595
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	22,340	18,002
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	243,672	275,570
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	219,152	248,993
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	208,150	241,195
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	5,116	4,974
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	5,886	2,824
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	75,061	85,731
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	73,011	81,220
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	70,974	81,220
Purchased loans | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	1,537	0
Purchased loans | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	500	0
Purchased loans | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	572,830	614,187
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	94,018	100,039
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	85,590	91,369
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	2,525	2,498
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	5,903	6,172
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	960,273	1,037,454
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	781,291	853,498
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	755,454	827,372
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	15,789	17,123
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	10,048	9,003
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	16,112	18,824
Purchased loans | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	665	681
Purchased loans | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	662	678
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	$ 3