Loans - Loans Individually Evaluated for Impairment Including Loans Modified in Troubled Debt Restructurings by Portfolio Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Total	$ 20,513	$ 18,501
Recorded Investment, Total	18,782	16,778
Allowance for Loan Losses Allocated	3,523	3,190
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	1,644	3,448
Recorded Investment, Related Allowance	1,644	3,448
Allowance for Loan Losses Allocated	313	1,126
Unpaid Principal Balance, No Related Allowance	2,669	3,716
Recorded Investment, No Related Allowance	1,771	2,788
Allowance for Loan Losses Allocated, No Related Allowance	0	0
Commercial Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	16,200	5,985
Recorded Investment, Related Allowance	15,367	5,343
Allowance for Loan Losses Allocated	3,210	2,025
Unpaid Principal Balance, No Related Allowance	0	5,001
Recorded Investment, No Related Allowance	0	4,951
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	351
Recorded Investment, Related Allowance	0	248
Allowance for Loan Losses Allocated	0	39
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

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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=66008989&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
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: 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=66008989&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=66008989&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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