Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Past due and nonaccrual loans
Loans, net of unearned income	$ 7,448,608	$ 6,202,709
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	1,008,935	717,490
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	577,720	552,679
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,295,852	1,878,177
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	3,390,389	2,898,895
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	175,712	155,468
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	2,155,141	1,489,137
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	2,134,864	1,457,912
Loans, net of unearned income	2,150,273	1,477,790
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	8,060	9,063
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	7,349	10,815
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,722	5,280
Loans, net of unearned income	4,868	11,347
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	338	677
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,808	5,390
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	301,100	128,200
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	299,473	125,417
Loans, net of unearned income	300,570	127,230
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	616	823
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	481	990
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	243	329
Loans, net of unearned income	530	970
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	260
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	287	381
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	100,082	68,753
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	100,082	67,760
Loans, net of unearned income	100,082	68,608
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	527
Purchased loans | Real estate – construction | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	321
Purchased loans | Real estate – construction | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	0	0
Loans, net of unearned income	0	145
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	145
Purchased loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	651,792	452,447
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	642,147	440,258
Loans, net of unearned income	648,091	448,212
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,650	4,572
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,294	3,382
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,140	1,771
Loans, net of unearned income	3,701	4,235
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	139	417
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,422	2,047
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	1,079,049	823,758
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,070,563	808,886
Loans, net of unearned income	1,078,577	818,043
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,448	3,045
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	4,566	6,112
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	177	3,054
Loans, net of unearned income	472	5,715
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	196	0
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	99	2,661
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	23,118	15,979
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	22,599	15,591
Loans, net of unearned income	22,953	15,697
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	346	96
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	8	10
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	162	126
Loans, net of unearned income	165	282
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3	0
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 0	$ 156