Non Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 7,767,657	$ 7,620,322
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	987,818	1,039,393
Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	712,818	633,389
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,433,099	2,343,721
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,461,174	3,427,530
Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	172,748	176,289
Non purchased loans and leases
Loan portfolio by risk-rating grades
Loans, net of unearned income	6,057,766	5,588,556
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,689,148	3,378,267
Non purchased loans and leases | Internal Noninvestment Grade | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	594,183	570,841
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	583,388	484,241
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	290,110	263,845
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,220,919	2,058,419
Non purchased loans and leases | Internal Noninvestment Grade | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	548	921
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,584,305	3,277,755
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	576,063	554,943
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	575,005	483,498
Non purchased loans and leases | Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	281,968	254,643
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,150,721	1,983,750
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	548	921
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Loans, net of unearned income	73,894	63,091
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	13,105	11,496
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	8,258	662
Non purchased loans and leases | Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	1,159	505
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	51,372	50,428
Non purchased loans and leases | Watch | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	0	0
Non purchased loans and leases | Substandard
Loan portfolio by risk-rating grades
Loans, net of unearned income	30,949	37,421
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	5,015	4,402
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	125	81
Non purchased loans and leases | Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	6,983	8,697
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	18,826	24,241
Non purchased loans and leases | Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 0	$ 0