Non Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Past due and nonaccrual loans
Total loans, gross	$ 7,701,432	$ 7,623,663
Unearned income	(3,362)	(3,341)
Loans, net of unearned income	7,698,070	7,620,322
Commercial, financial, agricultural
Past due and nonaccrual loans
Total loans, gross	1,046,818	1,039,393
Loans, net of unearned income	1,046,818	1,039,393
Lease financing
Past due and nonaccrual loans
Total loans, gross	55,898	57,354
Real estate – construction
Past due and nonaccrual loans
Total loans, gross	657,491	633,389
Loans, net of unearned income	657,491	633,389
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Total loans, gross	2,358,101	2,343,721
Loans, net of unearned income	2,358,101	2,343,721
Real estate – commercial mortgage
Past due and nonaccrual loans
Total loans, gross	3,463,953	3,427,530
Loans, net of unearned income	3,463,953	3,427,530
Installment loans to individuals
Past due and nonaccrual loans
Total loans, gross	119,171	122,276
Loans, net of unearned income	171,707	176,289
Non purchased loans and leases
Past due and nonaccrual loans
Total loans, gross	5,833,484	5,591,897
Unearned income	(3,362)	(3,341)
Loans, net of unearned income	5,830,122	5,588,556
Non purchased loans and leases | Commercial, financial, agricultural
Past due and nonaccrual loans
Total loans, gross	803,146	763,823
Non purchased loans and leases | Lease financing
Past due and nonaccrual loans
Total loans, gross	55,898	57,354
Non purchased loans and leases | Real estate – construction
Past due and nonaccrual loans
Total loans, gross	582,430	547,658
Non purchased loans and leases | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Total loans, gross	1,785,271	1,729,534
Non purchased loans and leases | Real estate – commercial mortgage
Past due and nonaccrual loans
Total loans, gross	2,503,680	2,390,076
Non purchased loans and leases | Installment loans to individuals
Past due and nonaccrual loans
Total loans, gross	103,059	103,452
Non purchased loans and leases | Accruing Loans
Past due and nonaccrual loans
Current Loans	5,797,686	5,558,312
Unearned income	(3,362)	(3,341)
Loans, net of unearned income	5,820,719	5,578,306
Non purchased loans and leases | Accruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Current Loans	796,898	759,143
Total loans, gross	800,982	761,887
Non purchased loans and leases | Accruing Loans | Lease financing
Past due and nonaccrual loans
Current Loans	55,215	57,148
Total loans, gross	55,739	57,195
Non purchased loans and leases | Accruing Loans | Real estate – construction
Past due and nonaccrual loans
Current Loans	578,816	547,608
Total loans, gross	582,430	547,658
Non purchased loans and leases | Accruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Current Loans	1,771,834	1,712,982
Total loans, gross	1,782,822	1,726,986
Non purchased loans and leases | Accruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Current Loans	2,495,780	2,381,871
Total loans, gross	2,499,085	2,384,519
Non purchased loans and leases | Accruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Current Loans	102,505	102,901
Total loans, gross	103,023	103,402
Non purchased loans and leases | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,712	4,129
Unearned income	0	0
Loans, net of unearned income	9,403	10,250
Non purchased loans and leases | Nonaccruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Current Loans	101	698
Total loans, gross	2,164	1,936
Non purchased loans and leases | Nonaccruing Loans | Lease financing
Past due and nonaccrual loans
Current Loans	0	0
Total loans, gross	159	159
Non purchased loans and leases | Nonaccruing Loans | Real estate – construction
Past due and nonaccrual loans
Current Loans	0	0
Total loans, gross	0	0
Non purchased loans and leases | Nonaccruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Current Loans	814	730
Total loans, gross	2,449	2,548
Non purchased loans and leases | Nonaccruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Current Loans	1,778	2,680
Total loans, gross	4,595	5,557
Non purchased loans and leases | Nonaccruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Current Loans	19	21
Total loans, gross	36	50
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans
Past due and nonaccrual loans
Loans past due	19,428	16,979
Unearned income	0	0
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans past due	3,078	2,722
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Lease financing
Past due and nonaccrual loans
Loans past due	481	47
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Real estate – construction
Past due and nonaccrual loans
Loans past due	3,564	50
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans past due	8,812	11,810
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans past due	3,016	1,921
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Loans past due	477	429
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans
Past due and nonaccrual loans
Loans past due	1,126	206
Unearned income	0	0
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans past due	508	205
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans | Lease financing
Past due and nonaccrual loans
Loans past due	0	0
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans | Real estate – construction
Past due and nonaccrual loans
Loans past due	0	0
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans past due	54	0
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans past due	564	0
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Loans past due	0	1
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans
Past due and nonaccrual loans
Loans past due	3,605	3,015
Unearned income	0	0
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans past due	1,006	22
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Lease financing
Past due and nonaccrual loans
Loans past due	43	0
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Real estate – construction
Past due and nonaccrual loans
Loans past due	50	0
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans past due	2,176	2,194
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans past due	289	727
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Loans past due	41	72
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans
Past due and nonaccrual loans
Loans past due	5,565	5,915
Unearned income	0	0
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans past due	1,555	1,033
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans | Lease financing
Past due and nonaccrual loans
Loans past due	159	159
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans | Real estate – construction
Past due and nonaccrual loans
Loans past due	0	0
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans past due	1,581	1,818
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans past due	2,253	2,877
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Loans past due	$ 17	$ 28