Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Balance	$ 77		$ 77		$ 83
Unpaid Principal Balance	77		77		83
Specific Allowance	19		19		73
Average Investment in Impaired Loans	78	$ 95	80	$ 159
Interest Income Recognized	1	4	3	9
Commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	811		811		619
Unpaid Principal Balance	811		811		727
Specific Allowance	85		85		0
Average Investment in Impaired Loans	846	1,127	853	1,103
Interest Income Recognized	10	5	16	22
Residential real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	0		0		0
Unpaid Principal Balance	0		0		0
Specific Allowance	0		0		0
Average Investment in Impaired Loans	0	0	0	0
Interest Income Recognized	0	0	0	0
Installment
Financing Receivable, Impaired [Line Items]
Recorded Balance	96		96		306
Unpaid Principal Balance	96		96		306
Specific Allowance	0		0		0
Average Investment in Impaired Loans	97	306	99	313
Interest Income Recognized	1	0	3	3
Loans without a specific valuation allowance
Financing Receivable, Impaired [Line Items]
Recorded Balance	566		566		598
Unpaid Principal Balance	566		566		706
Specific Allowance	0		0		0
Average Investment in Impaired Loans	594	963	605	982
Interest Income Recognized	10	0	19	13
Loans without a specific valuation allowance \| Commercial
Financing Receivable, Impaired [Line Items]
Recorded Balance	58		58		83
Unpaid Principal Balance	58		58		83
Specific Allowance	0		0		0
Average Investment in Impaired Loans	59	65	60	62
Interest Income Recognized	0	0	2	2
Loans without a specific valuation allowance \| Commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	412		412		209
Unpaid Principal Balance	412		412		317
Specific Allowance	0		0		0
Average Investment in Impaired Loans	438	592	446	607
Interest Income Recognized	9	3	14	8
Loans without a specific valuation allowance \| Residential real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	0		0		0
Unpaid Principal Balance	0		0		0
Specific Allowance	0		0		0
Average Investment in Impaired Loans	0	0	0	0
Interest Income Recognized	0	0	0	0
Loans without a specific valuation allowance \| Installment
Financing Receivable, Impaired [Line Items]
Recorded Balance	96		96		306
Unpaid Principal Balance	96		96		306
Specific Allowance	0		0		0
Average Investment in Impaired Loans	97	306	99	313
Interest Income Recognized	1	0	3	3
Loans with a specific valuation allowance
Financing Receivable, Impaired [Line Items]
Recorded Balance	418		418		410
Unpaid Principal Balance	418		418		410
Specific Allowance	104		104		73
Average Investment in Impaired Loans	427	565	427	593
Interest Income Recognized	2	6	3	21
Loans with a specific valuation allowance \| Commercial
Financing Receivable, Impaired [Line Items]
Recorded Balance	19		19		0
Unpaid Principal Balance	19		19		0
Specific Allowance	19		19		0
Average Investment in Impaired Loans	19	30	20	97
Interest Income Recognized	1	4	1	7
Loans with a specific valuation allowance \| Commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	399		399		410
Unpaid Principal Balance	399		399		410
Specific Allowance	85		85		73
Average Investment in Impaired Loans	408	535	407	496
Interest Income Recognized	1	2	2	14
Loans with a specific valuation allowance \| Residential real estate
Financing Receivable, Impaired [Line Items]
Recorded Balance	0		0		0
Unpaid Principal Balance	0		0		0
Specific Allowance	0		0		0
Average Investment in Impaired Loans	0	0	0	0
Interest Income Recognized	0	0	0	0
Loans with a specific valuation allowance \| Installment
Financing Receivable, Impaired [Line Items]
Recorded Balance	0		0		0
Unpaid Principal Balance	0		0		0
Specific Allowance	0		0		$ 0
Average Investment in Impaired Loans	0	0	0	0
Interest Income Recognized	$ 0	$ 0	$ 0	$ 0

A valuation allowance for financing receivables that are expected to be uncollectible.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-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://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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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.

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+ References

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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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

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