Non 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
Non purchased loans and leases
Loan portfolio by risk-rating grades
Total	5,293,467	4,713,572
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	3,188,818	2,808,367
Non purchased loans and leases | Internal Noninvestment Grade | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	527,641	439,704
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Total	415,386	402,549
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	256,854	200,385
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,988,433	1,765,729
Non purchased loans and leases | Internal Noninvestment Grade | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	504
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Total	3,150,037	2,761,884
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	522,410	434,323
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Total	415,168	402,156
Non purchased loans and leases | Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	247,584	190,882
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,964,371	1,734,523
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	504
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Total	21,821	26,416
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	4,505	4,531
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Total	130	393
Non purchased loans and leases | Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	3,778	3,374
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	13,408	18,118
Non purchased loans and leases | Watch | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	0
Non purchased loans and leases | Substandard
Loan portfolio by risk-rating grades
Total	16,960	20,067
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	726	850
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Total	88	0
Non purchased loans and leases | Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	5,492	6,129
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	10,654	$ 13,088
Non purchased loans and leases | Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	$ 0