Purchased Loans - Loan Portfolio Not Subject to Risk Rating (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loan portfolio not subject to risk rating
Total	$ 7,698,070	$ 7,620,322
Commercial, financial, agricultural
Loan portfolio not subject to risk rating
Total	1,046,818	1,039,393
Real estate – construction
Loan portfolio not subject to risk rating
Total	657,491	633,389
Real estate – 1-4 family mortgage
Loan portfolio not subject to risk rating
Total	2,358,101	2,343,721
Real estate – commercial mortgage
Loan portfolio not subject to risk rating
Total	3,463,953	3,427,530
Installment loans to individuals
Loan portfolio not subject to risk rating
Total	171,707	176,289
Purchased loans
Loan portfolio not subject to risk rating
Total	1,867,948	2,031,766
Purchased loans | Performing and Nonperforming
Loan portfolio not subject to risk rating
Total	485,501	520,075
Purchased loans | Performing
Loan portfolio not subject to risk rating
Total	483,627	518,604
Purchased loans | Non-Performing
Loan portfolio not subject to risk rating
Total	1,874	1,471
Purchased loans | Commercial, financial, agricultural
Loan portfolio not subject to risk rating
Total	243,672	275,570
Purchased loans | Commercial, financial, agricultural | Performing and Nonperforming
Loan portfolio not subject to risk rating
Total	11,587	11,262
Purchased loans | Commercial, financial, agricultural | Performing
Loan portfolio not subject to risk rating
Total	11,548	11,216
Purchased loans | Commercial, financial, agricultural | Non-Performing
Loan portfolio not subject to risk rating
Total	39	46
Purchased loans | Real estate – construction
Loan portfolio not subject to risk rating
Total	75,061	85,731
Purchased loans | Real estate – construction | Performing and Nonperforming
Loan portfolio not subject to risk rating
Total	2,050	4,511
Purchased loans | Real estate – construction | Performing
Loan portfolio not subject to risk rating
Total	2,050	4,511
Purchased loans | Real estate – construction | Non-Performing
Loan portfolio not subject to risk rating
Total	0	0
Purchased loans | Real estate – 1-4 family mortgage
Loan portfolio not subject to risk rating
Total	572,830	614,187
Purchased loans | Real estate – 1-4 family mortgage | Performing and Nonperforming
Loan portfolio not subject to risk rating
Total	428,616	460,179
Purchased loans | Real estate – 1-4 family mortgage | Performing
Loan portfolio not subject to risk rating
Total	427,099	459,038
Purchased loans | Real estate – 1-4 family mortgage | Non-Performing
Loan portfolio not subject to risk rating
Total	1,517	1,141
Purchased loans | Real estate – commercial mortgage
Loan portfolio not subject to risk rating
Total	960,273	1,037,454
Purchased loans | Real estate – commercial mortgage | Performing and Nonperforming
Loan portfolio not subject to risk rating
Total	29,435	27,618
Purchased loans | Real estate – commercial mortgage | Performing
Loan portfolio not subject to risk rating
Total	29,313	27,495
Purchased loans | Real estate – commercial mortgage | Non-Performing
Loan portfolio not subject to risk rating
Total	122	123
Purchased loans | Installment loans to individuals
Loan portfolio not subject to risk rating
Total	16,112	18,824
Purchased loans | Installment loans to individuals | Performing and Nonperforming
Loan portfolio not subject to risk rating
Total	13,813	16,505
Purchased loans | Installment loans to individuals | Performing
Loan portfolio not subject to risk rating
Total	13,617	16,344
Purchased loans | Installment loans to individuals | Non-Performing
Loan portfolio not subject to risk rating
Total	$ 196	$ 161