Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Troubled Debt Restructuring Related to Loans Held for Investment, Excluding Covered Loans, by Loan Type (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable Modifications [Line Items]
Nonaccrual	$ 67,836	$ 67,575
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	2,262	2,105
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	15,164	19,022
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	5,389	12,608
Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	33,870	33,338
Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	228	135
Other Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	1,419	155
Troubled Debt Restructurings [Member]
Financing Receivable Modifications [Line Items]
Accruing	2,346		$ 440
Nonaccrual	33,617	$ 23,200	23,939
Total	35,963		24,379
Troubled Debt Restructurings [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	26		272
Total	26		272
Troubled Debt Restructurings [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	563		14
Nonaccrual	3,052		3,077
Total	3,615		3,091
Troubled Debt Restructurings [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		426
Nonaccrual	360		436
Total	360		862
Troubled Debt Restructurings [Member] | Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	1,783		0
Nonaccrual	29,636		20,153
Total	31,419		20,153
Troubled Debt Restructurings [Member] | Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	3		1
Total	3		$ 1
Troubled Debt Restructurings [Member] | Other Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0
Nonaccrual	540
Total	$ 540