Loans, net and allowance for loan losses - Summarize Information in Concerning to Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	$ 6,800	$ 14,834	$ 8,066
Unpaid Principal Balance, With no related allowance, Total	9,303	20,780	10,714
Average Recorded Investment, With no related allowance, Total	7,433	16,701	13,438
Interest Income Recognized, With no related allowance, Total	40		265
Recorded Investment, With an allowance recorded, Total	7,293	3,393	6,525
Unpaid Principal Balance, With an allowance recorded, Total	7,293	3,393	6,525
Related Allowance, With an allowance recorded, Total	3,131	1,832	2,959
Average Recorded Investment, With an allowance recorded, Total	6,909	3,886	4,680
Interest Income Recognized, With an allowance recorded, Total	41	44	97
Recorded Investment, Total	14,093	18,227	14,591
Unpaid Principal Balance, Total	16,596	24,173	17,239
Related Allowance, With an allowance recorded, Total	3,131	1,832	2,959
Average Recorded Investment, Total	14,342	20,587	18,118
Interest Income Recognized, Total	81	44	362
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,319	2,008	2,379
Unpaid Principal Balance, With no related allowance, Total	3,900	4,762	4,084
Average Recorded Investment, With no related allowance, Total	2,349	3,493	2,669
Interest Income Recognized, With no related allowance, Total	20		141
Recorded Investment, With an allowance recorded, Total	1,625	1,728	1,569
Unpaid Principal Balance, With an allowance recorded, Total	1,625	1,728	1,569
Related Allowance, With an allowance recorded, Total	1,466	1,128	1,240
Average Recorded Investment, With an allowance recorded, Total	1,597	1,964	1,787
Interest Income Recognized, With an allowance recorded, Total	14	26	58
Recorded Investment, Total	3,944	3,736	3,948
Unpaid Principal Balance, Total	5,525	6,490	5,653
Related Allowance, With an allowance recorded, Total	1,466	1,128	1,240
Average Recorded Investment, Total	3,946	5,457	4,456
Interest Income Recognized, Total	34	26	199
Real Estate Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,198	9,958	2,932
Unpaid Principal Balance, With no related allowance, Total	2,936	12,746	3,690
Average Recorded Investment, With no related allowance, Total	2,565	10,227	7,944
Interest Income Recognized, With no related allowance, Total	19		120
Recorded Investment, With an allowance recorded, Total	3,939	1,404	3,525
Unpaid Principal Balance, With an allowance recorded, Total	3,939	1,404	3,525
Related Allowance, With an allowance recorded, Total	732	542	912
Average Recorded Investment, With an allowance recorded, Total	3,732	1,399	2,293
Interest Income Recognized, With an allowance recorded, Total	17	18	28
Recorded Investment, Total	6,137	11,362	6,457
Unpaid Principal Balance, Total	6,875	14,150	7,215
Related Allowance, With an allowance recorded, Total	732	542	912
Average Recorded Investment, Total	6,297	11,626	10,237
Interest Income Recognized, Total	36	18	148
Real Estate Residential [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	2,272	2,740	2,672
Unpaid Principal Balance, With no related allowance, Total	2,456	3,144	2,857
Average Recorded Investment, With no related allowance, Total	2,472	2,872	2,731
Interest Income Recognized, With no related allowance, Total	1		4
Recorded Investment, With an allowance recorded, Total	1,639	261	1,387
Unpaid Principal Balance, With an allowance recorded, Total	1,639	261	1,387
Related Allowance, With an allowance recorded, Total	843	162	769
Average Recorded Investment, With an allowance recorded, Total	1,513	523	590
Interest Income Recognized, With an allowance recorded, Total	10		10
Recorded Investment, Total	3,911	3,001	4,059
Unpaid Principal Balance, Total	4,095	3,405	4,244
Related Allowance, With an allowance recorded, Total	843	162	769
Average Recorded Investment, Total	3,985	3,395	3,321
Interest Income Recognized, Total	11		14
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance, Total	11	128	83
Unpaid Principal Balance, With no related allowance, Total	11	128	83
Average Recorded Investment, With no related allowance, Total	47	109	94
Recorded Investment, With an allowance recorded, Total	90		44
Unpaid Principal Balance, With an allowance recorded, Total	90		44
Related Allowance, With an allowance recorded, Total	90		38
Average Recorded Investment, With an allowance recorded, Total	67		10
Interest Income Recognized, With an allowance recorded, Total			1
Recorded Investment, Total	101	128	127
Unpaid Principal Balance, Total	101	128	127
Related Allowance, With an allowance recorded, Total	90		38
Average Recorded Investment, Total	114	109	104
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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