Non 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
Non purchased loans and leases
Loan portfolio by risk-rating grades
Total	5,830,122	5,588,556
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	3,562,164	3,378,267
Non purchased loans and leases | Internal Noninvestment Grade | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	602,988	570,841
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Total	521,733	484,241
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	270,385	263,845
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	2,166,206	2,058,419
Non purchased loans and leases | Internal Noninvestment Grade | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	852	921
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Total	3,456,223	3,277,755
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	585,850	554,943
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Total	512,603	483,498
Non purchased loans and leases | Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	262,107	254,643
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	2,094,811	1,983,750
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	852	921
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Total	73,146	63,091
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	11,380	11,496
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Total	8,690	662
Non purchased loans and leases | Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	669	505
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	52,407	50,428
Non purchased loans and leases | Watch | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	0	0
Non purchased loans and leases | Substandard
Loan portfolio by risk-rating grades
Total	32,795	37,421
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	5,758	4,402
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Total	440	81
Non purchased loans and leases | Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	7,609	8,697
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	18,988	24,241
Non purchased loans and leases | Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	$ 0	$ 0