Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 79,290
Provision for loan losses, LHFI	1,611	$ 3,961
Balance at end of period	79,005
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	79,290	76,733
Loans charged-off	(4,033)	(2,542)
Recoveries	2,137	3,083
Net (charge-offs) recoveries	(1,896)	541
Provision for loan losses, LHFI	1,611	3,961
Balance at end of period	79,005	81,235
Allowance for Loan Losses, LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	7,390	7,865
Loans charged-off	(35)	(2)
Recoveries	306	195
Provision for loan losses, LHFI	(137)	(232)
Balance at end of period	7,524	7,826
Allowance for Loan Losses, LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	8,641	10,874
Loans charged-off	(161)	(780)
Recoveries	216	267
Provision for loan losses, LHFI	(315)	(770)
Balance at end of period	8,381	9,591
Allowance for Loan Losses, LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	22,376	23,428
Recoveries	15	21
Provision for loan losses, LHFI	(912)	1,071
Balance at end of period	21,479	24,520
Allowance for Loan Losses, LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	3,450	2,790
Recoveries	10	6
Provision for loan losses, LHFI	(383)	(487)
Balance at end of period	3,077	2,309
Allowance for Loan Losses, LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	27,359	22,851
Loans charged-off	(1,859)	(121)
Recoveries	137	1,213
Provision for loan losses, LHFI	3,565	5,074
Balance at end of period	29,202	29,017
Allowance for Loan Losses, LHFI [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	2,890	3,470
Loans charged-off	(793)	(434)
Recoveries	534	501
Provision for loan losses, LHFI	517	(310)
Balance at end of period	3,148	3,227
Allowance for Loan Losses, LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	990	789
Provision for loan losses, LHFI	(198)	3
Balance at end of period	792	792
Allowance for Loan Losses, LHFI [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	6,194	4,666
Loans charged-off	(1,185)	(1,205)
Recoveries	919	880
Provision for loan losses, LHFI	(526)	(388)
Balance at end of period	$ 5,402	$ 3,953