Loans, net and allowance for loan losses - Major Classification of Loans by Past Due Status (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 16,934	$ 21,260
Current	1,542,933	1,511,705
Total Loans	1,559,867	1,532,965
Loans > 90 Days and Accruing	462	844
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	5,327	8,093
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,884	1,215
Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	9,723	11,952
Commercial
Financing Receivable, Recorded Investment, Past Due
Total Past Due	2,005	1,258
Current	429,159	407,556
Total Loans	431,164	408,814
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	440	249
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	33	75
Commercial | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,532	934
Real estate Commercial
Financing Receivable, Recorded Investment, Past Due
Total Past Due	6,845	12,325
Current	696,349	687,819
Total Loans	703,194	700,144
Loans > 90 Days and Accruing	108
Real estate Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,257	4,782
Real estate Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	779	527
Real estate Commercial | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	4,809	7,016
Real estate Residential
Financing Receivable, Recorded Investment, Past Due
Total Past Due	6,615	6,015
Current	279,774	283,766
Total Loans	286,389	289,781
Loans > 90 Days and Accruing	124	558
Real estate Residential | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	2,766	2,100
Real estate Residential | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	904	354
Real estate Residential | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	2,945	3,561
Consumer
Financing Receivable, Recorded Investment, Past Due
Total Past Due	1,469	1,662
Current	137,651	132,564
Total Loans	139,120	134,226
Loans > 90 Days and Accruing	230	286
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	864	962
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due
Total Past Due	168	259
Consumer | Greater than 90 Days
Financing Receivable, Recorded Investment, Past Due
Total Past Due	$ 437	$ 441