Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Past due and nonaccrual loans
Loans, net of unearned income	$ 7,698,070	$ 7,620,322
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	1,046,818	1,039,393
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	657,491	633,389
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,358,101	2,343,721
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	3,463,953	3,427,530
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	171,707	176,289
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	1,867,948	2,031,766
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,848,899	2,010,852
Loans, net of unearned income	1,862,608	2,027,342
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	9,145	10,759
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	4,564	5,731
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,827	2,612
Loans, net of unearned income	5,340	4,424
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	1,271	635
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,242	1,177
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	243,672	275,570
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	242,313	273,488
Loans, net of unearned income	243,253	275,139
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	388	1,119
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	552	532
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	105	232
Loans, net of unearned income	419	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	314	199
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	75,061	85,731
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	75,061	85,316
Loans, net of unearned income	75,061	85,731
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	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	0	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	0	0
Purchased loans | Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	572,830	614,187
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	561,608	602,464
Loans, net of unearned income	569,215	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	5,491	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	2,116	2,280
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,304	2,109
Loans, net of unearned income	3,615	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	1,265	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	1,046	879
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	960,273	1,037,454
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	954,128	1,031,141
Loans, net of unearned income	959,126	1,036,998
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,142	2,947
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,856	2,910
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	317	166
Loans, net of unearned income	1,147	456
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	191
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	830	99
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	16,112	18,824
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	15,789	18,443
Loans, net of unearned income	15,953	18,660
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	124	208
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	40	9
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	101	105
Loans, net of unearned income	159	164
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	6	59
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 52	$ 0