Purchased Loans - Aging of Past Due and Nonaccrual Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Past due and nonaccrual loans
Loans, net of unearned income	$ 9,054,654	$ 9,083,129
Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	1,305,076	1,295,912
Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	781,531	740,668
Real estate – 1-4 family mortgage
Past due and nonaccrual loans
Loans, net of unearned income	2,765,472	2,795,343
Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	4,017,969	4,051,509
Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	184,606	199,697
Purchased loans
Past due and nonaccrual loans
Loans, net of unearned income	2,350,366	2,693,417
Purchased loans | Accruing Loans
Past due and nonaccrual loans
Current Loans	2,322,669	2,661,269
Loans, net of unearned income	2,343,116	2,687,581
Purchased loans | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	12,760	19,080
Purchased loans | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	7,687	7,232
Purchased loans | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	3,011	1,849
Loans, net of unearned income	7,250	5,836
Purchased loans | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	588	204
Purchased loans | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	3,651	3,783
Purchased loans | Commercial, financial, agricultural
Past due and nonaccrual loans
Loans, net of unearned income	374,478	420,263
Purchased loans | Commercial, financial, agricultural | Accruing Loans
Past due and nonaccrual loans
Current Loans	368,967	417,786
Loans, net of unearned income	373,518	419,694
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,755	1,811
Purchased loans | Commercial, financial, agricultural | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	796	97
Purchased loans | Commercial, financial, agricultural | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	322	92
Loans, net of unearned income	960	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	638	477
Purchased loans | Real estate – construction
Past due and nonaccrual loans
Loans, net of unearned income	65,402	105,149
Purchased loans | Real estate – construction | Accruing Loans
Past due and nonaccrual loans
Current Loans	65,295	103,846
Loans, net of unearned income	65,402	105,149
Purchased loans | Real estate – construction | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	107	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	604,855	707,453
Purchased loans | Real estate – 1-4 family mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	592,308	690,697
Loans, net of unearned income	600,432	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	5,080	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	3,044	4,455
Purchased loans | Real estate – 1-4 family mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	2,105	1,237
Loans, net of unearned income	4,423	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	480	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	1,838	1,881
Purchased loans | Real estate – commercial mortgage
Past due and nonaccrual loans
Loans, net of unearned income	1,276,567	1,423,144
Purchased loans | Real estate – commercial mortgage | Accruing Loans
Past due and nonaccrual loans
Current Loans	1,267,915	1,413,346
Loans, net of unearned income	1,275,066	1,421,467
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	3,404	5,711
Purchased loans | Real estate – commercial mortgage | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	3,747	2,410
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	331	276
Loans, net of unearned income	1,501	1,677
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	108	0
Purchased loans | Real estate – commercial mortgage | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	1,062	1,401
Purchased loans | Installment loans to individuals
Past due and nonaccrual loans
Loans, net of unearned income	29,064	37,408
Purchased loans | Installment loans to individuals | Accruing Loans
Past due and nonaccrual loans
Current Loans	28,184	35,594
Loans, net of unearned income	28,698	37,138
Purchased loans | Installment loans to individuals | Accruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	414	1,342
Purchased loans | Installment loans to individuals | Accruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	100	202
Purchased loans | Installment loans to individuals | Nonaccruing Loans
Past due and nonaccrual loans
Current Loans	253	244
Loans, net of unearned income	366	270
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 30-89 Days Past Due
Past due and nonaccrual loans
Loans past due	0	2
Purchased loans | Installment loans to individuals | Nonaccruing Loans | 90 Days or More Past Due
Past due and nonaccrual loans
Loans past due	$ 113	$ 24