Non Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 9,088,293	$ 9,083,129
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	1,308,457	1,295,912
Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	741,073	740,668
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,769,173	2,795,343
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,083,632	4,051,509
Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	185,958	199,697
Non purchased loans and leases
Loan portfolio by risk-rating grades
Loans, net of unearned income	6,565,599	6,389,712
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,997,486	3,829,377
Non purchased loans and leases | Internal Noninvestment Grade | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	702,163	641,102
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	584,263	568,968
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	327,919	330,484
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,383,141	2,288,823
Non purchased loans and leases | Internal Noninvestment Grade | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	0	0
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,873,695	3,705,961
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	675,107	615,803
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	570,637	558,494
Non purchased loans and leases | Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	319,715	321,564
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,308,236	2,210,100
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	0	0
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Loans, net of unearned income	75,966	79,882
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	15,523	18,326
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	5,469	2,317
Non purchased loans and leases | Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,619	4,660
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	50,355	54,579
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	47,825	43,534
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	11,533	6,973
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	8,157	8,157
Non purchased loans and leases | Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,585	4,260
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	24,550	24,144
Non purchased loans and leases | Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 0	$ 0