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	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable Modifications [Line Items]
Nonaccrual	$ 61,363	$ 67,575
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	2,566	2,105
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	16,753	19,022
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	7,997	12,608
Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	32,548	33,338
Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	174	135
Other Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	1,256	155
Troubled Debt Restructurings [Member]
Financing Receivable Modifications [Line Items]
Accruing	1,234		$ 0
Nonaccrual	33,218	$ 23,200	13,240
Total	34,452		13,240
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	208		641
Total	208		641
Troubled Debt Restructurings [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	501		0
Nonaccrual	3,407		3,589
Total	3,908		3,589
Troubled Debt Restructurings [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	370		802
Total	370		802
Troubled Debt Restructurings [Member] | Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	733		0
Nonaccrual	28,673		8,207
Total	29,406		8,207
Troubled Debt Restructurings [Member] | Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	4		1
Total	4		$ 1
Troubled Debt Restructurings [Member] | Other Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0
Nonaccrual	556
Total	$ 556