Loans, net and allowance for loan losses - Summarize Information in Concerning to Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	$ 8,066	$ 18,568	$ 1,408
Unpaid Principal Balance, With no related allowance, Total	10,714	26,353	1,408
Average Recorded Investment, With no related allowance, Total	13,438	18,628	1,573
Interest Income Recognized, With no related allowance, Total	265
Recorded Investment, With an allowance recorded, Total	6,525	4,378	3,863
Unpaid Principal Balance, With an allowance recorded, Total	6,525	4,378	3,863
Related Allowance, With an allowance recorded, Total	2,959	2,024	1,226
Average Recorded Investment, With an allowance recorded, Total	4,680	4,263	3,495
Interest Income Recognized, With an allowance recorded, Total	97	184	175
Recorded Investment, Total	14,591	22,946	5,271
Unpaid Principal Balance, Total	17,239	30,731	5,271
Related Allowance, With an allowance recorded, Total	2,959	2,024	1,226
Average Recorded Investment, Total	18,118	22,891	5,068
Interest Income Recognized, Total	362	184	175
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,379	4,978	304
Unpaid Principal Balance, With no related allowance, Total	4,084	9,474	304
Average Recorded Investment, With no related allowance, Total	2,669	5,824	152
Interest Income Recognized, With no related allowance, Total	141
Recorded Investment, With an allowance recorded, Total	1,569	2,200	351
Unpaid Principal Balance, With an allowance recorded, Total	1,569	2,200	351
Related Allowance, With an allowance recorded, Total	1,240	1,500	351
Average Recorded Investment, With an allowance recorded, Total	1,787	2,182	351
Interest Income Recognized, With an allowance recorded, Total	58	95	17
Recorded Investment, Total	3,948	7,178	655
Unpaid Principal Balance, Total	5,653	11,674	655
Related Allowance, With an allowance recorded, Total	1,240	1,500	351
Average Recorded Investment, Total	4,456	8,006	503
Interest Income Recognized, Total	199	95	17
Real Estate Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,932	10,496	145
Unpaid Principal Balance, With no related allowance, Total	3,690	13,352	145
Average Recorded Investment, With no related allowance, Total	7,944	10,095	201
Interest Income Recognized, With no related allowance, Total	120
Recorded Investment, With an allowance recorded, Total	3,525	1,394	2,015
Unpaid Principal Balance, With an allowance recorded, Total	3,525	1,394	2,015
Related Allowance, With an allowance recorded, Total	912	300	550
Average Recorded Investment, With an allowance recorded, Total	2,293	1,409	2,104
Interest Income Recognized, With an allowance recorded, Total	28	76	114
Recorded Investment, Total	6,457	11,890	2,160
Unpaid Principal Balance, Total	7,215	14,746	2,160
Related Allowance, With an allowance recorded, Total	912	300	550
Average Recorded Investment, Total	10,237	11,504	2,305
Interest Income Recognized, Total	148	76	114
Real Estate Residential [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,672	3,004	928
Unpaid Principal Balance, With no related allowance, Total	2,857	3,437	928
Average Recorded Investment, With no related allowance, Total	2,731	2,614	1,152
Interest Income Recognized, With no related allowance, Total	4
Recorded Investment, With an allowance recorded, Total	1,387	784	1,497
Unpaid Principal Balance, With an allowance recorded, Total	1,387	784	1,497
Related Allowance, With an allowance recorded, Total	769	224	325
Average Recorded Investment, With an allowance recorded, Total	590	672	1,040
Interest Income Recognized, With an allowance recorded, Total	10	13	44
Recorded Investment, Total	4,059	3,788	2,425
Unpaid Principal Balance, Total	4,244	4,221	2,425
Related Allowance, With an allowance recorded, Total	769	224	325
Average Recorded Investment, Total	3,321	3,286	2,192
Interest Income Recognized, Total	14	13	44
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	83	90	31
Unpaid Principal Balance, With no related allowance, Total	83	90	31
Average Recorded Investment, With no related allowance, Total	94	95	68
Recorded Investment, With an allowance recorded, Total	44
Unpaid Principal Balance, With an allowance recorded, Total	44
Related Allowance, With an allowance recorded, Total	38
Average Recorded Investment, With an allowance recorded, Total	10
Interest Income Recognized, With an allowance recorded, Total	1
Recorded Investment, Total	127	90	31
Unpaid Principal Balance, Total	127	90	31
Related Allowance, With an allowance recorded, Total	38
Average Recorded Investment, Total	104	95	68
Interest Income Recognized, Total	$ 1

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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 interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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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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Amount of allowance for credit losses related to recorded investment.

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Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

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