Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Past due and nonaccrual loans
Loans, net of unearned income	$ 9,122,907	$ 7,620,322
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	1,313,344	1,039,393
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	736,985	633,389
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,762,683	2,343,721
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	4,112,585	3,427,530
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	197,310	176,289
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	2,912,669	2,031,766
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	2,879,067	2,010,852
Loans, net of unearned income	2,907,860	2,027,342
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	20,833	10,759
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	7,960	5,731
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,801	2,612
Loans, net of unearned income	4,809	4,424
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	819	635
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,189	1,177
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	495,545	275,570
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	489,237	273,488
Loans, net of unearned income	495,232	275,139
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	5,508	1,119
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	487	532
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	74	232
Loans, net of unearned income	313	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	239	199
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	112,093	85,731
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	109,153	85,316
Loans, net of unearned income	111,829	85,731
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	2,676	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	264	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	264	0
Purchased loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	761,913	614,187
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	748,488	602,464
Loans, net of unearned income	758,873	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	6,737	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	3,648	2,280
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,330	2,109
Loans, net of unearned income	3,040	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	353	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	1,357	879
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	1,503,075	1,037,454
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,493,220	1,031,141
Loans, net of unearned income	1,502,127	1,036,998
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	5,140	2,947
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	3,767	2,910
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	207	166
Loans, net of unearned income	948	456
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	412	191
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	329	99
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	40,043	18,824
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	38,969	18,443
Loans, net of unearned income	39,799	18,660
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	772	208
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	58	9
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	190	105
Loans, net of unearned income	244	164
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	54	59
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 0	$ 0