Loans, net and allowance for loan losses - Allocation of Allowance for Loan Losses and Related Loans by Major Classification of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Allowance for loan losses:
Ending balance	$ 10,803	$ 10,338	$ 8,859	$ 8,651
Ending balance: individually evaluated for impairment	2,821	2,682
Ending balance: collectively evaluated for impairment	7,672	7,379
Ending balance: loans acquired with deteriorated credit quality	310	277
Loans receivable:
Ending balance	1,237,168	1,209,894
Ending balance: individually evaluated for impairment	11,342	11,807
Ending balance: collectively evaluated for impairment	1,223,075	1,195,303
Ending balance: loans acquired with deteriorated credit quality	2,751	2,784
Commercial [Member]
Allowance for loan losses:
Ending balance	2,420	2,321	1,961	2,008
Ending balance: individually evaluated for impairment	1,263	1,072
Ending balance: collectively evaluated for impairment	954	1,081
Ending balance: loans acquired with deteriorated credit quality	203	168
Loans receivable:
Ending balance	319,468	319,590
Ending balance: individually evaluated for impairment	2,631	2,595
Ending balance: collectively evaluated for impairment	315,524	315,642
Ending balance: loans acquired with deteriorated credit quality	1,313	1,353
Real Estate Commercial [Member]
Allowance for loan losses:
Ending balance	3,087	3,037	2,473	2,394
Ending balance: individually evaluated for impairment	625	805
Ending balance: collectively evaluated for impairment	2,355	2,125
Ending balance: loans acquired with deteriorated credit quality	107	107
Loans receivable:
Ending balance	522,432	493,481
Ending balance: individually evaluated for impairment	4,757	5,084
Ending balance: collectively evaluated for impairment	516,295	487,024
Ending balance: loans acquired with deteriorated credit quality	1,380	1,373
Real Estate Residential [Member]
Allowance for loan losses:
Ending balance	3,909	3,690	3,248	3,135
Ending balance: individually evaluated for impairment	843	767
Ending balance: collectively evaluated for impairment	3,066	2,921
Ending balance: loans acquired with deteriorated credit quality		2
Loans receivable:
Ending balance	305,663	310,667
Ending balance: individually evaluated for impairment	3,853	4,001
Ending balance: collectively evaluated for impairment	301,752	306,608
Ending balance: loans acquired with deteriorated credit quality	58	58
Consumer [Member]
Allowance for loan losses:
Ending balance	1,387	1,290	1,177	1,114
Ending balance: individually evaluated for impairment	90	38
Ending balance: collectively evaluated for impairment	1,297	1,252
Loans receivable:
Ending balance	89,605	86,156
Ending balance: individually evaluated for impairment	101	127
Ending balance: collectively evaluated for impairment	$ 89,504	$ 86,029

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

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

-Publisher FASB

-Name Accounting Standards Codification

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

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

-Publisher FASB

-Name Accounting Standards Codification

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

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+ References

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

-Publisher FASB

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- Definition

Amount before allowance and after 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. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

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-SubTopic 10

-Section 50

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-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

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