Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Past due and nonaccrual loans
Total loans, gross	$ 7,623,663	$ 6,205,118
Unearned income	(3,341)	(2,409)
Loans, net of unearned income	7,620,322	6,202,709
Commercial, financial, agricultural
Past due and nonaccrual loans
Total loans, gross	1,039,393	717,490
Loans, net of unearned income	1,039,393	717,490
Lease financing
Past due and nonaccrual loans
Total loans, gross	57,354	49,250
Real estate – construction
Past due and nonaccrual loans
Total loans, gross	633,389	552,679
Loans, net of unearned income	633,389	552,679
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Total loans, gross	2,343,721	1,878,177
Loans, net of unearned income	2,343,721	1,878,177
Real estate – commercial mortgage
Past due and nonaccrual loans
Total loans, gross	3,427,530	2,898,895
Loans, net of unearned income	3,427,530	2,898,895
Installment loans to individuals
Past due and nonaccrual loans
Total loans, gross	122,276	108,627
Loans, net of unearned income	176,289	155,468
Purchased
Past due and nonaccrual loans
Total loans, gross	2,031,766	1,489,137
Unearned income	0	0
Loans, net of unearned income	2,031,766	1,489,137
Purchased | Accruing Loans
Past due and nonaccrual loans
Current Loans	2,010,852	1,457,912
Unearned income	0	0
Loans, net of unearned income	2,027,342	1,477,790
Purchased | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	10,759	9,063
Purchased | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	5,731	10,815
Purchased | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,612	5,280
Unearned income	0	0
Loans, net of unearned income	4,424	11,347
Purchased | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	635	677
Purchased | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,177	5,390
Purchased | Commercial, financial, agricultural
Past due and nonaccrual loans
Total loans, gross	275,570	128,200
Purchased | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	273,488	125,417
Total loans, gross	275,139	127,230
Purchased | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	1,119	823
Purchased | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	532	990
Purchased | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	232	329
Total loans, gross	431	970
Purchased | Commercial, financial, agricultural | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	260
Purchased | Commercial, financial, agricultural | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	199	381
Purchased | Lease financing
Past due and nonaccrual loans
Total loans, gross	0	0
Purchased | Lease financing | Accruing Loans
Past due and nonaccrual loans
Current Loans	0	0
Total loans, gross	0	0
Purchased | Lease financing | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased | Lease financing | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased | Lease financing | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	0	0
Total loans, gross	0	0
Purchased | Lease financing | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased | Lease financing | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased | Real estate – construction
Past due and nonaccrual loans
Total loans, gross	85,731	68,753
Purchased | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	85,316	67,760
Total loans, gross	85,731	68,608
Purchased | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	415	527
Purchased | Real estate – construction | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	321
Purchased | Real estate – construction | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	0	0
Total loans, gross	0	145
Purchased | Real estate – construction | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased | Real estate – construction | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	145
Purchased | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Total loans, gross	614,187	452,447
Purchased | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	602,464	440,258
Total loans, gross	610,814	448,212
Purchased | Real estate – 1-4 family mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	6,070	4,572
Purchased | Real estate – 1-4 family mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,280	3,382
Purchased | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,109	1,771
Total loans, gross	3,373	4,235
Purchased | Real estate – 1-4 family mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	385	417
Purchased | Real estate – 1-4 family mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	879	2,047
Purchased | Real estate – commercial mortgage
Past due and nonaccrual loans
Total loans, gross	1,037,454	823,758
Purchased | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,031,141	808,886
Total loans, gross	1,036,998	818,043
Purchased | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	2,947	3,045
Purchased | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,910	6,112
Purchased | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	166	3,054
Total loans, gross	456	5,715
Purchased | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	191	0
Purchased | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	99	2,661
Purchased | Installment loans to individuals
Past due and nonaccrual loans
Total loans, gross	18,824	15,979
Purchased | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	18,443	15,591
Total loans, gross	18,660	15,697
Purchased | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	208	96
Purchased | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	9	10
Purchased | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	105	126
Total loans, gross	164	282
Purchased | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	59	0
Purchased | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 0	$ 156