Loans, net and allowance for loan losses - Major Classification of Loans by Past Due Status (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 14,540	$ 21,260
Current	1,582,822	1,511,705
Total Loans	1,597,362	1,532,965
Loans > 90 Days and Accruing	835	844
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	4,432	8,093
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	996	1,215
Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	9,112	11,952
Commercial
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,891	1,258
Current	450,289	407,556
Total Loans	452,180	408,814
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	501	249
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	105	75
Commercial | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,285	934
Real estate Commercial
Financing Receivable, Recorded Investment, Past Due
Total Past Due	6,189	12,325
Current	713,463	687,819
Total Loans	719,652	700,144
Real estate Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	2,003	4,782
Real estate Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	71	527
Real estate Commercial | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	4,115	7,016
Real estate Residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	4,650	6,015
Current	281,582	283,766
Total Loans	286,232	289,781
Loans > 90 Days and Accruing	567	558
Real estate Residential | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	964	2,100
Real estate Residential | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	474	354
Real estate Residential | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	3,212	3,561
Consumer
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,810	1,662
Current	137,488	132,564
Total Loans	139,298	134,226
Loans > 90 Days and Accruing	268	286
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	964	962
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	346	259
Consumer | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 500	$ 441