Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	$ 60,202	$ 60,387
Total LHFI With No Related Allowance Recorded	23,648	42,674
Total LHFI With an Allowance Recorded	28,807	6,262
Total LHFI Carrying Amount	52,455	48,936
Related Allowance	15,749	1,743
Average Recorded Investment	45,230	38,005
Commercial and Industrial Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	39,845	36,874
Total LHFI With No Related Allowance Recorded	14,657	31,281
Total LHFI With an Allowance Recorded	19,045	518
Total LHFI Carrying Amount	33,702	31,799
Related Allowance	13,869	131
Average Recorded Investment	28,218	22,734
Consumer Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	3	17
Total LHFI With No Related Allowance Recorded	0	0
Total LHFI With an Allowance Recorded	3	17
Total LHFI Carrying Amount	3	17
Related Allowance	0	0
Average Recorded Investment	6	9
State and Other Political Subdivision Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	8,765	0
Total LHFI With No Related Allowance Recorded	4,079	0
Total LHFI With an Allowance Recorded	4,683	0
Total LHFI Carrying Amount	8,762	0
Related Allowance	683	0
Average Recorded Investment	4,381	0
Other Loans [Member]
Loan and Lease Receivables, Impaired [Abstract]
Total LHFI Unpaid Principal Balance	1,432	556
Total LHFI With No Related Allowance Recorded	230	0
Total LHFI With an Allowance Recorded	1,076	556
Total LHFI Carrying Amount	1,306	556
Related Allowance	1,076	41
Average Recorded Investment	816	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	1,801	1,704
Total LHFI With No Related Allowance Recorded	1,546	1,206
Total LHFI With an Allowance Recorded	26	199
Total LHFI Carrying Amount	1,572	1,405
Related Allowance	0	75
Average Recorded Investment	1,747	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	4,771	6,031
Total LHFI With No Related Allowance Recorded	126	160
Total LHFI With an Allowance Recorded	3,614	4,576
Total LHFI Carrying Amount	3,740	4,736
Related Allowance	43	1,331
Average Recorded Investment	4,217	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	3,585	15,205
Total LHFI With No Related Allowance Recorded	3,010	10,027
Total LHFI With an Allowance Recorded	360	396
Total LHFI Carrying Amount	3,370	10,423
Related Allowance	78	165
Average Recorded Investment	5,845	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://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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