Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Past due and nonaccrual loans
Loans, net of unearned income	$ 9,313,784	$ 9,083,129
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	1,328,560	1,295,912
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	816,695	740,668
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,797,633	2,795,343
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	4,022,375	4,051,509
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	348,521	199,697
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	2,281,966	2,693,417
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	2,252,340	2,661,269
Loans, net of unearned income	2,275,843	2,687,581
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	16,469	19,080
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	7,034	7,232
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,435	1,849
Loans, net of unearned income	6,123	5,836
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	357	204
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	3,331	3,783
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	339,693	420,263
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	334,410	417,786
Loans, net of unearned income	338,219	419,694
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	2,133	1,811
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,676	97
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	290	92
Loans, net of unearned income	1,474	569
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	1,184	477
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	52,106	105,149
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	51,731	103,846
Loans, net of unearned income	52,106	105,149
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	375	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	561,725	707,453
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	549,220	690,697
Loans, net of unearned income	557,992	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	5,829	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	2,943	4,455
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	1,548	1,237
Loans, net of unearned income	3,733	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	333	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	1,852	1,881
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	1,212,905	1,423,144
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,206,299	1,413,346
Loans, net of unearned income	1,212,318	1,421,467
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,674	5,711
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	2,345	2,410
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	333	276
Loans, net of unearned income	587	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	254	1,401
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	115,537	37,408
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	110,680	35,594
Loans, net of unearned income	115,208	37,138
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	4,458	1,342
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	70	202
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	264	244
Loans, net of unearned income	329	270
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	24	2
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 41	$ 24