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, 2018	Dec. 31, 2017
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	$ 60,633	$ 60,387
Total LHFI With No Related Allowance Recorded	22,135	42,674
Total LHFI With an Allowance Recorded	27,495	6,262
Total LHFI Carrying Amount	49,630	48,936
Related Allowance	9,034	1,743
Average Recorded Investment	43,817	38,005
Commercial and Industrial Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	37,306	36,874
Total LHFI With No Related Allowance Recorded	16,018	31,281
Total LHFI With an Allowance Recorded	15,681	518
Total LHFI Carrying Amount	31,699	31,799
Related Allowance	6,771	131
Average Recorded Investment	27,216	22,734
Consumer Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	1	17
Total LHFI With No Related Allowance Recorded	0	0
Total LHFI With an Allowance Recorded	1	17
Total LHFI Carrying Amount	1	17
Related Allowance	0	0
Average Recorded Investment	5	9
State and Other Political Subdivision Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	0	0
Total LHFI With No Related Allowance Recorded	0	0
Total LHFI With an Allowance Recorded	0	0
Total LHFI Carrying Amount	0	0
Related Allowance	0	0
Average Recorded Investment	0	0
Other Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	1,206	556
Total LHFI With No Related Allowance Recorded	0	0
Total LHFI With an Allowance Recorded	1,116	556
Total LHFI Carrying Amount	1,116	556
Related Allowance	1,116	41
Average Recorded Investment	721	325
Loans Secured by Real Estate [Member] \| Construction, Land Development and Other Land [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	1,157	1,704
Total LHFI With No Related Allowance Recorded	743	1,206
Total LHFI With an Allowance Recorded	189	199
Total LHFI Carrying Amount	932	1,405
Related Allowance	64	75
Average Recorded Investment	1,427	1,923
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	4,630	6,031
Total LHFI With No Related Allowance Recorded	719	160
Total LHFI With an Allowance Recorded	2,977	4,576
Total LHFI Carrying Amount	3,696	4,736
Related Allowance	42	1,331
Average Recorded Investment	4,194	4,693
Loans Secured by Real Estate [Member] \| Secured by Nonfarm, Nonresidential Properties [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	16,333	15,205
Total LHFI With No Related Allowance Recorded	4,655	10,027
Total LHFI With an Allowance Recorded	7,531	396
Total LHFI Carrying Amount	12,186	10,423
Related Allowance	1,041	165
Average Recorded Investment	10,254	8,321
Loans Secured by Real Estate [Member] \| Other Real Estate Secured [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	0	0
Total LHFI With No Related Allowance Recorded	0	0
Total LHFI With an Allowance Recorded	0	0
Total LHFI Carrying Amount	0	0
Related Allowance	0	0
Average Recorded Investment	$ 0	$ 0

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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