Non Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Past due and nonaccrual loans
Total loans, gross	$ 9,126,211	$ 7,623,663
Unearned income	(3,304)	(3,341)
Loans, net of unearned income	9,122,907	7,620,322
Commercial, financial, agricultural
Past due and nonaccrual loans
Total loans, gross	1,313,344	1,039,393
Loans, net of unearned income	1,313,344	1,039,393
Lease financing
Past due and nonaccrual loans
Total loans, gross	57,576	57,354
Real estate – construction
Past due and nonaccrual loans
Total loans, gross	736,985	633,389
Loans, net of unearned income	736,985	633,389
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Total loans, gross	2,762,683	2,343,721
Loans, net of unearned income	2,762,683	2,343,721
Real estate – commercial mortgage
Past due and nonaccrual loans
Total loans, gross	4,112,585	3,427,530
Loans, net of unearned income	4,112,585	3,427,530
Installment loans to individuals
Past due and nonaccrual loans
Total loans, gross	143,038	122,276
Loans, net of unearned income	197,310	176,289
Non purchased loans and leases
Past due and nonaccrual loans
Total loans, gross	6,213,542	5,591,897
Unearned income	(3,304)	(3,341)
Loans, net of unearned income	6,210,238	5,588,556
Non purchased loans and leases | Commercial, financial, agricultural
Past due and nonaccrual loans
Total loans, gross	817,799	763,823
Non purchased loans and leases | Lease financing
Past due and nonaccrual loans
Total loans, gross	57,576	57,354
Non purchased loans and leases | Real estate – construction
Past due and nonaccrual loans
Total loans, gross	624,892	547,658
Non purchased loans and leases | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Total loans, gross	2,000,770	1,729,534
Non purchased loans and leases | Real estate – commercial mortgage
Past due and nonaccrual loans
Total loans, gross	2,609,510	2,390,076
Non purchased loans and leases | Installment loans to individuals
Past due and nonaccrual loans
Total loans, gross	102,995	103,452
Non purchased loans and leases | Accruing Loans
Past due and nonaccrual loans
Current Loans	6,181,873	5,558,312
Unearned income	(3,304)	(3,341)
Loans, net of unearned income	6,200,542	5,578,306
Non purchased loans and leases | Accruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Current Loans	814,246	759,143
Total loans, gross	816,171	761,887
Non purchased loans and leases | Accruing Loans | Lease financing
Past due and nonaccrual loans
Current Loans	57,256	57,148
Total loans, gross	57,576	57,195
Non purchased loans and leases | Accruing Loans | Real estate – construction
Past due and nonaccrual loans
Current Loans	623,823	547,608
Total loans, gross	624,892	547,658
Non purchased loans and leases | Accruing Loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Current Loans	1,986,079	1,712,982
Total loans, gross	1,996,972	1,726,986
Non purchased loans and leases | Accruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Current Loans	2,601,728	2,381,871
Total loans, gross	2,605,288	2,384,519
Non purchased loans and leases | Accruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Current Loans	102,045	102,901
Total loans, gross	102,947	103,402
Non purchased loans and leases | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,692	4,129
Unearned income	0	0
Loans, net of unearned income	9,696	10,250
Non purchased loans and leases | Nonaccruing Loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Current Loans	205	698
Total loans, gross	1,628	1,936
Non purchased loans and leases | Nonaccruing Loans | Lease financing
Past due and nonaccrual loans
Current Loans	0	0
Total loans, gross	0	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	913	730
Total loans, gross	3,798	2,548
Non purchased loans and leases | Nonaccruing Loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Current Loans	1,570	2,680
Total loans, gross	4,222	5,557
Non purchased loans and leases | Nonaccruing Loans | Installment loans to individuals
Past due and nonaccrual loans
Current Loans	4	21
Total loans, gross	48	50
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans
Past due and nonaccrual loans
Loans past due	14,863	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	1,606	2,722
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Lease financing
Past due and nonaccrual loans
Loans past due	320	47
Non purchased loans and leases | 30-89 Days Past Due | Accruing Loans | Real estate – construction
Past due and nonaccrual loans
Loans past due	1,069	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	7,928	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	3,080	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	860	429
Non purchased loans and leases | 30-89 Days Past Due | Nonaccruing Loans
Past due and nonaccrual loans
Loans past due	537	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	0	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	207	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	324	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	6	1
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans
Past due and nonaccrual loans
Loans past due	3,806	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	319	22
Non purchased loans and leases | 90 Days or More Past Due | Accruing Loans | Lease financing
Past due and nonaccrual loans
Loans past due	0	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	0	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	2,965	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	480	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	42	72
Non purchased loans and leases | 90 Days or More Past Due | Nonaccruing Loans
Past due and nonaccrual loans
Loans past due	6,467	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	1,423	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	0	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	2,678	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	2,328	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	$ 38	$ 28