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Allowance for Loan Losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 3,947	$ 5,219	$ 4,575
Recorded Investment with No Allowance	3,530	4,428	3,811
Recorded Investment with Allowance	0	85	0
Total Recorded Investment	3,530	4,513	3,811
Related Allowance	0	31	0
Average Recorded Investment	3,894	4,756	4,656
Interest Income Recognized	48	49	232
Real estate loans | Construction and Land Development
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	992	2,455	1,627
Recorded Investment with No Allowance	874	2,022	1,150
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	874	2,022	1,150
Related Allowance	0	0	0
Average Recorded Investment	1,257	1,922	1,814
Interest Income Recognized	13	13	63
Real estate loans | Secured by 1-4 Family Residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	1,417	1,469	1,387
Recorded Investment with No Allowance	1,332	1,358	1,307
Recorded Investment with Allowance	0	85	0
Total Recorded Investment	1,332	1,443	1,307
Related Allowance	0	31	0
Average Recorded Investment	1,299	1,781	1,637
Interest Income Recognized	15	16	64
Real estate loans | Other real estate loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	1,464	1,206	1,483
Recorded Investment with No Allowance	1,263	977	1,289
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	1,263	977	1,289
Related Allowance	0	0	0
Average Recorded Investment	1,274	980	1,137
Interest Income Recognized	19	19	95
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	74	89	78
Recorded Investment with No Allowance	61	71	65
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	61	71	65
Related Allowance	0	0	0
Average Recorded Investment	64	73	68
Interest Income Recognized	$ 1	$ 1	$ 10

X

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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+ References

No definition available.

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The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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Average 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 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 interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

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)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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+ 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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+ References

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-Topic 310
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-Section 50
-Paragraph 14A
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