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, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 76,733
Provision for loan losses, LHFI	3,961	$ 2,762
Balance at end of period	81,235
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	76,733	71,265
Loans charged-off	(2,542)	(4,202)
Recoveries	3,083	2,620
Net (charge-offs) recoveries	541	(1,582)
Provision for loan losses, LHFI	3,961	2,762
Balance at end of period	81,235	72,445
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	7,865	9,085
Loans charged-off	(2)	(58)
Recoveries	195	303
Provision for loan losses, LHFI	(232)	(804)
Balance at end of period	7,826	8,526
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	10,874	10,347
Loans charged-off	(780)	(241)
Recoveries	267	152
Provision for loan losses, LHFI	(770)	529
Balance at end of period	9,591	10,787
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	23,428	20,967
Recoveries	21	182
Provision for loan losses, LHFI	1,071	759
Balance at end of period	24,520	21,908
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	2,790	2,263
Recoveries	6	20
Provision for loan losses, LHFI	(487)	856
Balance at end of period	2,309	3,139
Allowance for Loan Losses, LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	22,851	22,011
Loans charged-off	(121)	(1,984)
Recoveries	1,213	488
Provision for loan losses, LHFI	5,074	1,136
Balance at end of period	29,017	21,651
Allowance for Loan Losses, LHFI [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	3,470	3,241
Loans charged-off	(434)	(745)
Recoveries	501	480
Provision for loan losses, LHFI	(310)	216
Balance at end of period	3,227	3,192
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	789	859
Provision for loan losses, LHFI	3	(11)
Balance at end of period	792	848
Allowance for Loan Losses, LHFI [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	4,666	2,492
Loans charged-off	(1,205)	(1,174)
Recoveries	880	995
Provision for loan losses, LHFI	(388)	81
Balance at end of period	$ 3,953	$ 2,394