Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Past due and nonaccrual loans
Loans, net of unearned income	$ 9,088,293	$ 9,083,129
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	1,308,457	1,295,912
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	741,073	740,668
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,769,173	2,795,343
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	4,083,632	4,051,509
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	185,958	199,697
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	2,522,694	2,693,417
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	2,488,384	2,661,269
Loans, net of unearned income	2,514,866	2,687,581
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	21,046	19,080
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	5,436	7,232
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	3,027	1,849
Loans, net of unearned income	7,828	5,836
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	611	204
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	4,190	3,783
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	387,376	420,263
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	379,993	417,786
Loans, net of unearned income	386,196	419,694
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	5,596	1,811
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	607	97
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	346	92
Loans, net of unearned income	1,180	569
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	311	0
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	523	477
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	89,954	105,149
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	87,696	103,846
Loans, net of unearned income	89,954	105,149
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	2,258	1,235
Purchased loans | Real estate – construction | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	0	68
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	654,265	707,453
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	637,630	690,697
Loans, net of unearned income	649,914	704,133
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	9,631	8,981
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,653	4,455
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,973	1,237
Loans, net of unearned income	4,351	3,320
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	299	202
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,079	1,881
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	1,357,446	1,423,144
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,351,020	1,413,346
Loans, net of unearned income	1,355,528	1,421,467
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	2,605	5,711
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,903	2,410
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	458	276
Loans, net of unearned income	1,918	1,677
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	0
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,460	1,401
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	33,653	37,408
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	32,045	35,594
Loans, net of unearned income	33,274	37,138
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	956	1,342
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	273	202
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	250	244
Loans, net of unearned income	379	270
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	1	2
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 128	$ 24