Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	$ 805	$ 745
Allowance for loan losses, Collectively evaluated for impairment	13,681	13,958
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	3,196	2,670
Total ending allowance balance	17,682	17,373	16,674	16,234
Loans:
Loans individually evaluated for impairment	4,581	4,545
Loans collectively evaluated for impairment	1,336,272	1,335,674
Acquired with deteriorated credit quality	713	719
Loans modified in a troubled debt restructuring	18,409	18,658
Total ending loans balance	1,359,975us-gaap_LoansReceivableNet	1,359,596us-gaap_LoansReceivableNet
Commercial, Industrial, and Agricultural [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	205	254
Allowance for loan losses, Collectively evaluated for impairment	6,240	6,703
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	778	157
Total ending allowance balance	7,223	7,114	8,278	8,212
Loans:
Loans individually evaluated for impairment	3,434	3,394
Loans collectively evaluated for impairment	428,991	421,144
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	3,880	3,920
Total ending loans balance	436,305	428,458
Commercial Mortgages [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	407	294
Allowance for loan losses, Collectively evaluated for impairment	2,471	2,503
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	2,418	2,513
Total ending allowance balance	5,296	5,310	3,704	3,536
Loans:
Loans individually evaluated for impairment	490	494
Loans collectively evaluated for impairment	329,829	336,801
Acquired with deteriorated credit quality	713	719
Loans modified in a troubled debt restructuring	14,529	14,738
Total ending loans balance	345,561	352,752
Residential Real Estate [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	193	197
Allowance for loan losses, Collectively evaluated for impairment	2,593	2,282
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,786	2,479	2,455	2,450
Loans:
Loans individually evaluated for impairment	657	657
Loans collectively evaluated for impairment	503,912	501,660
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	504,569	502,317
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	2,129	2,205
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	2,129	2,205	1,972	1,763
Loans:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	67,461	69,648
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	67,461	69,648
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	74	71
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	74	71	68	66
Loans:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	5,004	5,233
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	5,004	5,233
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	174	194
Acquired with deteriorated credit quality	0	0
Allowance for loan losses, Modified in a troubled debt restructuring	0	0
Total ending allowance balance	174	194	197	207
Loans:
Loans individually evaluated for impairment	0	0
Loans collectively evaluated for impairment	1,075	1,188
Acquired with deteriorated credit quality	0	0
Loans modified in a troubled debt restructuring	0	0
Total ending loans balance	$ 1,075	$ 1,188

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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.

+ References

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

-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

-Name Accounting Standards Codification

-Topic 310

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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

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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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-Publisher FASB

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-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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-Name Accounting Standards Codification

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-SubTopic 210

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