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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Financing Receivable Modifications [Line Items]
Nonaccrual	$ 56,423	$ 61,624
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	1,346	2,218
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	17,089	14,718
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	8,440	9,621
Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	18,127	23,938
Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	147	205
Other Loans [Member]
Financing Receivable Modifications [Line Items]
Nonaccrual	1,277	1,402
Troubled Debt Restructurings [Member]
Financing Receivable Modifications [Line Items]
Accruing	13,844		$ 60
Nonaccrual	16,388	$ 16,700	25,786
Total	30,232		25,846
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	22		189
Total	22		189
Troubled Debt Restructurings [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable Modifications [Line Items]
Accruing	58		60
Nonaccrual	3,098		2,916
Total	3,156		2,976
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	339		380
Total	339		380
Troubled Debt Restructurings [Member] | Commercial and Industrial Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	13,786		0
Nonaccrual	12,380		21,745
Total	26,166		21,745
Troubled Debt Restructurings [Member] | Consumer Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0
Nonaccrual	31
Total	31
Troubled Debt Restructurings [Member] | Other Loans [Member]
Financing Receivable Modifications [Line Items]
Accruing	0		0
Nonaccrual	518		556
Total	$ 518		$ 556