Loans - Loans Individually Evaluated for Impairment Including Loans Modified in Troubled Debt Restructurings by Portfolio Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Total	$ 22,683	$ 20,330
Recorded Investment, Total	20,670	18,541
Allowance for Loan Losses Allocated	6,468	5,736
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	1,861	1,915
Recorded Investment, Related Allowance	1,852	1,915
Allowance for Loan Losses Allocated	298	292
Unpaid Principal Balance, No Related Allowance	5,478	5,264
Recorded Investment, No Related Allowance	4,716	4,485
Allowance for Loan Losses Allocated, No Related Allowance	0	0
Commercial Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	9,290	9,940
Recorded Investment, Related Allowance	9,007	9,731
Allowance for Loan Losses Allocated	6,170	5,444
Unpaid Principal Balance, No Related Allowance	6,054	3,211
Recorded Investment, No Related Allowance	5,095	2,410
Allowance for Loan Losses Allocated, No Related Allowance	0	0
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	0	0
Recorded Investment, Related Allowance	0	0
Allowance for Loan Losses Allocated	0	0
Unpaid Principal Balance, No Related Allowance	0	0
Recorded Investment, No Related Allowance	0	0
Allowance for Loan Losses Allocated, No Related Allowance	$ 0	$ 0

Impaired loans without allowance.

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- Definition

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+ 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://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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- Definition

Amount of allowance for credit losses related to recorded investment.

+ 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)(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://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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- Definition

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

+ References

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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://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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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://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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