Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	$ 745	$ 427
Allowance for loan losses, Collectively evaluated for impairment	13,958	14,352
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	2,670	1,455
Total ending allowance balance	17,373	16,234	14,060	12,615
Loans:
Loans individually evaluated for impairment	4,545	6,172
Loans collectively evaluated for impairment	1,335,674	1,278,726
Acquired with deteriorated credit quality	719	2,225
Loans modified in a troubled debt restructuring	18,658	12,136
Total ending loans balance	1,359,596us-gaap_LoansReceivableNet	1,299,259us-gaap_LoansReceivableNet
Commercial, Industrial, and Agricultural [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	254	372
Allowance for loan losses, Collectively evaluated for impairment	6,703	7,725
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	157	115
Total ending allowance balance	7,114	8,212	4,940	4,511
Loans:
Loans individually evaluated for impairment	3,394	4,923
Loans collectively evaluated for impairment	421,144	421,002
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	3,920	1,790
Total ending loans balance	428,458	427,715
Commercial Mortgages [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	294	55
Allowance for loan losses, Collectively evaluated for impairment	2,503	2,141
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	2,513	1,340
Total ending allowance balance	5,310	3,536	4,697	4,470
Loans:
Loans individually evaluated for impairment	494	1,249
Loans collectively evaluated for impairment	336,801	329,645
Acquired with deteriorated credit quality	719	2,225
Loans modified in a troubled debt restructuring	14,738	10,346
Total ending loans balance	352,752	343,465
Residential Real Estate [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	197	0
Allowance for loan losses, Collectively evaluated for impairment	2,282	2,450
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,479	2,450	2,466	1,991
Loans:
Loans individually evaluated for impairment	657	0
Loans collectively evaluated for impairment	501,660	459,114
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	502,317	459,114
Consumer [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	2,205	1,763
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,205	1,763	1,699	1,404
Loans:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	69,648	63,491
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	69,648	63,491
Credit Cards [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	71	66
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	71	66	83	71
Loans:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	5,233	5,065
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	5,233	5,065
Overdrafts [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0	0
Allowance for loan losses, Collectively evaluated for impairment	194	207
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	194	207	175	168
Loans:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	1,188	409
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	$ 1,188	$ 409

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Financing receivable allowance for credit losses modifications recorded investment.

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- Definition

The balance of financing receivables that were acquired with deteriorated credit quality.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

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- Definition

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

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-Publisher FASB

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The valuation allowance for financing receivables acquired with deteriorated credit quality.

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-Publisher FASB

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

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-Paragraph 11B

-Subparagraph (g)

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

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- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

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- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a

-Article 9

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