Loans and allowance for loan losses - Allocation of Allowance for Loan Losses by Loan Category Broken Out Between Loans Individually Evaluated for Impairment and Collectively Evaluated for Impairment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 268	$ 191	$ 513
Collectively evaluated for impairment	27,783	23,850	21,234
Acquired with deteriorated credit quality	881	0	0
Ending balance	28,932	24,041	21,747	$ 24,460
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	3	20	135
Collectively evaluated for impairment	5,247	4,441	5,174
Acquired with deteriorated credit quality	98	0	0
Ending balance	5,348	4,461	5,309	5,135
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	9,677	7,135	4,940
Acquired with deteriorated credit quality	52	0	0
Ending balance	9,729	7,135	4,940	5,143
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	7	18	23
Collectively evaluated for impairment	3,205	3,179	3,174
Acquired with deteriorated credit quality	216	0	0
Ending balance	3,428	3,197	3,197	4,176
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	811	944	1,613
Acquired with deteriorated credit quality	0	0	0
Ending balance	811	944	1,613	2,201
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	566	434	504
Acquired with deteriorated credit quality	0	0	0
Ending balance	566	434	504	311
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	53	120	113
Collectively evaluated for impairment	3,066	3,438	3,189
Acquired with deteriorated credit quality	13	0	0
Ending balance	3,132	3,558	3,302	3,682
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	205	33	242
Collectively evaluated for impairment	3,628	2,784	1,777
Acquired with deteriorated credit quality	316	0	0
Ending balance	4,149	2,817	2,019	2,622
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	1,583	1,495	863
Acquired with deteriorated credit quality	186	0	0
Ending balance	$ 1,769	$ 1,495	$ 863	$ 1,190