Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Past due and nonaccrual loans
Loans, net of unearned income	$ 7,767,657	$ 7,620,322
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	987,818	1,039,393
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	712,818	633,389
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,433,099	2,343,721
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	3,461,174	3,427,530
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	172,748	176,289
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	1,709,891	2,031,766
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,693,676	2,010,852
Loans, net of unearned income	1,705,330	2,027,342
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	6,163	10,759
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	5,491	5,731
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,059	2,612
Loans, net of unearned income	4,561	4,424
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	692	635
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,810	1,177
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	197,455	275,570
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	195,614	273,488
Loans, net of unearned income	197,022	275,139
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	894	1,119
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	514	532
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	84	232
Loans, net of unearned income	433	431
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	349	199
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	70,438	85,731
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	69,519	85,316
Loans, net of unearned income	70,438	85,731
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	919	415
Purchased loans | Real estate – construction | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased loans | Real estate – construction | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	0	0
Loans, net of unearned income	0	0
Purchased loans | Real estate – construction | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased loans | Real estate – construction | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	520,649	614,187
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	512,235	602,464
Loans, net of unearned income	517,539	610,814
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,127	6,070
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,177	2,280
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,614	2,109
Loans, net of unearned income	3,110	3,373
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	260	385
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,236	879
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	906,219	1,037,454
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	901,527	1,031,141
Loans, net of unearned income	905,447	1,036,998
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	1,150	2,947
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,770	2,910
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	210	166
Loans, net of unearned income	772	456
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	430	191
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	132	99
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	15,130	18,824
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	14,781	18,443
Loans, net of unearned income	14,884	18,660
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	73	208
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	30	9
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	151	105
Loans, net of unearned income	246	164
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	2	59
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 93	$ 0