Loans, net and allowance for loan losses - Major Classification of Loans by Past Due Status (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 17,226	$ 14,993
Current	1,708,555	1,678,072
Total Loans	1,725,781	1,693,065
Loans > 90 Days and Accruing	435	735
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	6,557	4,081
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,556	2,325
Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	9,113	8,587
Commercial
Financing Receivable, Recorded Investment, Past Due
Total Past Due	891	1,200
Current	466,513	474,999
Total Loans	467,404	476,199
Loans > 90 Days and Accruing	3
Commercial \| 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	99	124
Commercial \| 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	18	216
Commercial \| Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	774	860
Real estate Commercial
Financing Receivable, Recorded Investment, Past Due
Total Past Due	9,743	5,737
Current	823,754	780,473
Total Loans	833,497	786,210
Real estate Commercial \| 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	4,608	1,722
Real estate Commercial \| 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	416	194
Real estate Commercial \| Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	4,719	3,821
Real estate Residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	5,430	6,228
Current	283,937	281,707
Total Loans	289,367	287,935
Loans > 90 Days and Accruing	325	549
Real estate Residential \| 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,305	1,134
Real estate Residential \| 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	780	1,551
Real estate Residential \| Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	3,345	3,543
Consumer
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,162	1,828
Current	134,351	140,893
Total Loans	135,513	142,721
Loans > 90 Days and Accruing	107	186
Consumer \| 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	545	1,101
Consumer \| 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	342	364
Consumer \| Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 275	$ 363

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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- Definition

Financing receivables that are current.

+ References

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=84173941&loc=SL6953401-111524

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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
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

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