Loans, net and allowance for loan losses - Summarize Information in Concerning to Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	$ 10,320	$ 2,292	$ 10,320	$ 2,292	$ 18,568
Unpaid Principal Balance, With no related allowance, Total	13,030	2,292	13,030	2,292	26,353
Average Recorded Investment, With no related allowance, Total	12,844	2,012	14,868	1,794	18,628
Recorded Investment, With an allowance recorded, Total	6,441	4,087	6,441	4,087	4,378
Interest Income Recognized, With no related allowance, Total	40		132
Unpaid Principal Balance, With an allowance recorded, Total	6,441	4,087	6,441	4,087	4,378
Related Allowance, With an allowance recorded, Total	2,115	1,959	2,115	1,959	2,024
Average Recorded Investment, With an allowance recorded, Total	4,922	4,994	4,072	3,818	4,263
Interest Income Recognized, With an allowance recorded, Total	38	93	110	149	184
Recorded Investment, Total	16,761	6,379	16,761	6,379	22,946
Unpaid Principal Balance, Total	19,471	6,379	19,471	6,379	30,731
Related Allowance, With an allowance recorded, Total	2,115	1,959	2,115	1,959	2,024
Average Recorded Investment, Total	17,766	7,006	18,940	5,612	22,891
Interest Income Recognized, Total	78	93	242	149	184
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,352	305	2,352	305	4,978
Unpaid Principal Balance, With no related allowance, Total	4,076	305	4,076	305	9,474
Average Recorded Investment, With no related allowance, Total	2,501	82	2,771	213	5,824
Recorded Investment, With an allowance recorded, Total	1,781	2,064	1,781	2,064	2,200
Interest Income Recognized, With no related allowance, Total	19		71
Unpaid Principal Balance, With an allowance recorded, Total	1,781	2,064	1,781	2,064	2,200
Related Allowance, With an allowance recorded, Total	1,163	1,464	1,163	1,464	1,500
Average Recorded Investment, With an allowance recorded, Total	1,769	3,226	1,825	1,440	2,182
Interest Income Recognized, With an allowance recorded, Total	20	70	63	82	95
Recorded Investment, Total	4,133	2,369	4,133	2,369	7,178
Unpaid Principal Balance, Total	5,857	2,369	5,857	2,369	11,674
Related Allowance, With an allowance recorded, Total	1,163	1,464	1,163	1,464	1,500
Average Recorded Investment, Total	4,270	3,308	4,596	1,653	8,006
Interest Income Recognized, Total	39	70	134	82	95
Real Estate Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	5,275	267	5,275	267	10,496
Unpaid Principal Balance, With no related allowance, Total	6,074	267	6,074	267	13,352
Average Recorded Investment, With no related allowance, Total	7,587	293	9,238	289	10,095
Recorded Investment, With an allowance recorded, Total	3,893	1,424	3,893	1,424	1,394
Interest Income Recognized, With no related allowance, Total	20		58
Unpaid Principal Balance, With an allowance recorded, Total	3,893	1,424	3,893	1,424	1,394
Related Allowance, With an allowance recorded, Total	660	300	660	300	300
Average Recorded Investment, With an allowance recorded, Total	2,640	1,396	1,816	1,435	1,409
Interest Income Recognized, With an allowance recorded, Total	14	19	43	57	76
Recorded Investment, Total	9,168	1,691	9,168	1,691	11,890
Unpaid Principal Balance, Total	9,967	1,691	9,967	1,691	14,746
Related Allowance, With an allowance recorded, Total	660	300	660	300	300
Average Recorded Investment, Total	10,227	1,689	11,054	1,724	11,504
Interest Income Recognized, Total	34	19	101	57	76
Real Estate Residential [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,629	1,621	2,629	1,621	3,004
Unpaid Principal Balance, With no related allowance, Total	2,816	1,621	2,816	1,621	3,437
Average Recorded Investment, With no related allowance, Total	2,675	1,552	2,758	1,217	2,614
Recorded Investment, With an allowance recorded, Total	750	599	750	599	784
Interest Income Recognized, With no related allowance, Total	1		3
Unpaid Principal Balance, With an allowance recorded, Total	750	599	750	599	784
Related Allowance, With an allowance recorded, Total	275	195	275	195	224
Average Recorded Investment, With an allowance recorded, Total	504	372	428	943	672
Interest Income Recognized, With an allowance recorded, Total	4	4	4	10	13
Recorded Investment, Total	3,379	2,220	3,379	2,220	3,788
Unpaid Principal Balance, Total	3,566	2,220	3,566	2,220	4,221
Related Allowance, With an allowance recorded, Total	275	195	275	195	224
Average Recorded Investment, Total	3,179	1,924	3,186	2,160	3,286
Interest Income Recognized, Total	5	4	7	10	13
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	64	99	64	99	90
Unpaid Principal Balance, With no related allowance, Total	64	99	64	99	90
Average Recorded Investment, With no related allowance, Total	81	85	101	75	95
Recorded Investment, With an allowance recorded, Total	17		17
Interest Income Recognized, With no related allowance, Total
Unpaid Principal Balance, With an allowance recorded, Total	17		17
Related Allowance, With an allowance recorded, Total	17		17
Average Recorded Investment, With an allowance recorded, Total	9		3
Interest Income Recognized, With an allowance recorded, Total
Recorded Investment, Total	81	99	81	99	90
Unpaid Principal Balance, Total	81	99	81	99	90
Related Allowance, With an allowance recorded, Total	17		17
Average Recorded Investment, Total	90	85	104	75	95
Interest Income Recognized, Total

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Average 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 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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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=35738203&loc=SL6953550-111524

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