Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	$ 79,401	$ 62,921
Total LHFI With No Related Allowance Recorded	26,456	25,553
Total LHFI With an Allowance Recorded	25,559	30,282
Total LHFI Carrying Amount	52,015	55,835
Related Allowance	5,040	6,354
Average Recorded Investment	49,466	46,920
Commercial and Industrial Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	25,414	37,786
Total LHFI With No Related Allowance Recorded	12,890	12,893
Total LHFI With an Allowance Recorded	16,635	17,824
Total LHFI Carrying Amount	29,525	30,717
Related Allowance	3,407	4,334
Average Recorded Investment	28,125	26,725
Consumer Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	31	2
Total LHFI With No Related Allowance Recorded	0	0
Total LHFI With an Allowance Recorded	31	2
Total LHFI Carrying Amount	31	2
Related Allowance	0	0
Average Recorded Investment	18	6
State and Other Political Subdivision Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	8,611	8,688
Total LHFI With No Related Allowance Recorded	4,079	4,079
Total LHFI With an Allowance Recorded	4,370	4,516
Total LHFI Carrying Amount	8,449	8,595
Related Allowance	370	516
Average Recorded Investment	6,373	4,297
Other Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	1,392	1,418
Total LHFI With No Related Allowance Recorded	230	230
Total LHFI With an Allowance Recorded	1,006	1,052
Total LHFI Carrying Amount	1,236	1,282
Related Allowance	1,006	1,052
Average Recorded Investment	1,020	804
Loans Secured by Real Estate [Member] | Construction, Land Development and Other Land [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	952	1,794
Total LHFI With No Related Allowance Recorded	673	1,528
Total LHFI With an Allowance Recorded	22	24
Total LHFI Carrying Amount	695	1,552
Related Allowance	0	0
Average Recorded Investment	1,216	1,738
Loans Secured by Real Estate [Member] | Secured by 1-4 Family Residential Properties [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	34,862	4,951
Total LHFI With No Related Allowance Recorded	1,093	95
Total LHFI With an Allowance Recorded	3,156	3,868
Total LHFI Carrying Amount	4,249	3,963
Related Allowance	32	39
Average Recorded Investment	4,288	4,328
Loans Secured by Real Estate [Member] | Secured by Nonfarm, Nonresidential Properties [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	7,482	8,282
Total LHFI With No Related Allowance Recorded	6,599	6,728
Total LHFI With an Allowance Recorded	339	2,748
Total LHFI Carrying Amount	6,938	9,476
Related Allowance	225	413
Average Recorded Investment	7,918	8,898
Loans Secured by Real Estate [Member] | Other Real Estate Secured [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	657	0
Total LHFI With No Related Allowance Recorded	892	0
Total LHFI With an Allowance Recorded	0	248
Total LHFI Carrying Amount	892	248
Related Allowance	0	0
Average Recorded Investment	$ 508	$ 124