Loans, net and allowance for loan losses - Major Classification of Loans by Past Due Status (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	$ 7,000	$ 7,167
60-89 Days Past Due	2,491	1,478
Greater than 90 Days	10,322	16,321
Total Past Due	19,813	24,966
Current	1,190,081	1,151,651
Ending balance	1,209,894	1,176,617	623,530
Loans > 90 Days and Accruing	1,623	1,455
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	898	1,052
60-89 Days Past Due	117	105
Greater than 90 Days	1,322	2,041
Total Past Due	2,337	3,198
Current	317,253	347,482
Ending balance	319,590	350,680	91,724
Loans > 90 Days and Accruing		6
Real Estate Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,100	1,641
60-89 Days Past Due	888	75
Greater than 90 Days	3,868	9,372
Total Past Due	6,856	11,088
Current	486,625	401,970
Ending balance	493,481	413,058	217,496
Loans > 90 Days and Accruing	136	200
Real Estate Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	3,154	3,676
60-89 Days Past Due	1,239	985
Greater than 90 Days	4,585	4,247
Total Past Due	8,978	8,908
Current	301,689	313,154
Ending balance	310,667	322,062	261,912
Loans > 90 Days and Accruing	1,062	678
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	848	798
60-89 Days Past Due	247	313
Greater than 90 Days	547	661
Total Past Due	1,642	1,772
Current	84,514	89,045
Ending balance	86,156	90,817	52,398
Loans > 90 Days and Accruing	$ 425	$ 571

Financing receivables that are less than 60 days past due but more than 29 days past due.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7A

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524

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Financing receivables that are less than 90 days past due but more than 59 days past due.

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Recorded investment in financing receivables that are 90 days or more past due and still accruing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

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-Section 50

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Financing receivables that are current.

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Financing receivables that are equal to or greater than 90 days past due.

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Recorded Investment in financing receivables that are past due at the balance sheet date.

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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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-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

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

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