Loans - Loans Individually Evaluated for Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
With an allowance recorded:
Allowance for Loan Losses Allocated	$ 6,472	$ 5,736
With no related allowance recorded:
Unpaid Principal Balance, Total	22,195	20,330
Recorded Investment, Total	19,711	18,541
Commercial, Industrial, and Agricultural
With an allowance recorded:
Unpaid Principal Balance	1,096	1,915
Recorded Investment	1,096	1,915
Allowance for Loan Losses Allocated	237	292
With no related allowance recorded:
Unpaid Principal Balance	5,918	5,264
Recorded Investment	5,202	4,485
Commercial mortgages
With an allowance recorded:
Unpaid Principal Balance	9,717	9,940
Recorded Investment	8,991	9,731
Allowance for Loan Losses Allocated	6,235	5,444
With no related allowance recorded:
Unpaid Principal Balance	5,464	3,211
Recorded Investment	4,422	2,410
Residential Real Estate
With an allowance recorded:
Unpaid Principal Balance	0	0
Recorded Investment	0	0
Allowance for Loan Losses Allocated	0	0
With no related allowance recorded:
Unpaid Principal Balance	0	0
Recorded Investment	$ 0	$ 0

Impaired Financing Receivable, Unpaid Principal Balance With No Allowance Recorded [Abstract]

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- Definition

Impaired Financing Receivable, Unpaid Principal Balance With an Allowance Recorded [Abstract]

+ References

No definition available.

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- Definition

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 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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- Definition

Amount of allowance for credit losses related to recorded investment.

+ 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 (a)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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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.

+ 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 (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

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- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no 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 (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

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- Definition

Unpaid principal balance of impaired financing receivables for which there is 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 (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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