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, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable Modifications [Line Items]
Nonaccrual	$ 69,289	$ 49,234
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	2,365	3,323
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	18,593	20,329
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	17,611	8,482
Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	25,101	15,824
Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	607	300
Troubled Debt Restructurings [Member]
Financing Receivable Modifications [Line Items]
Accruing	440		$ 0
Nonaccrual	23,939	$ 14,400	3,669
Total	24,379		3,669
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	272		556
Total	272		556
Troubled Debt Restructurings [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	14		0
Nonaccrual	3,077		2,545
Total	3,091		2,545
Troubled Debt Restructurings [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	426		0
Nonaccrual	436		179
Total	862		179
Troubled Debt Restructurings [Member] | Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	20,153		387
Total	20,153		387
Troubled Debt Restructurings [Member] | Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	1		2
Total	$ 1		$ 2