Non Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 9,122,907	$ 7,620,322
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	1,313,344	1,039,393
Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	736,985	633,389
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,762,683	2,343,721
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,112,585	3,427,530
Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	197,310	176,289
Non purchased loans and leases
Loan portfolio by risk-rating grades
Loans, net of unearned income	6,210,238	5,588,556
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,752,988	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	613,790	570,841
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	558,539	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	305,021	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	2,275,068	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	570	921
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,643,101	3,277,755
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	596,176	554,943
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	548,080	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	298,125	254,643
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,200,150	1,983,750
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	570	921
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Loans, net of unearned income	67,587	63,091
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	12,885	11,496
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	764	662
Non purchased loans and leases | Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	1,093	505
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	52,845	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	42,300	37,421
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,729	4,402
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	9,695	81
Non purchased loans and leases | Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	5,803	8,697
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	22,073	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