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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 895	$ 268	$ 191
Collectively evaluated for impairment	28,571	27,783	23,850
Acquired with deteriorated credit quality	1,673	881	0
Ending balance	31,139	28,932	24,041	$ 21,747
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	241	3	20
Collectively evaluated for impairment	4,457	5,247	4,441
Acquired with deteriorated credit quality	107	98	0
Ending balance	4,805	5,348	4,461	5,309
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	10,192	9,677	7,135
Acquired with deteriorated credit quality	2	52	0
Ending balance	10,194	9,729	7,135	4,940
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	8	7	18
Collectively evaluated for impairment	2,940	3,205	3,179
Acquired with deteriorated credit quality	164	216	0
Ending balance	3,112	3,428	3,197	3,197
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	9	0	0
Collectively evaluated for impairment	743	811	944
Acquired with deteriorated credit quality	0	0	0
Ending balance	752	811	944	1,613
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	544	566	434
Acquired with deteriorated credit quality	0	0	0
Ending balance	544	566	434	504
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	238	53	120
Collectively evaluated for impairment	3,853	3,066	3,438
Acquired with deteriorated credit quality	18	13	0
Ending balance	4,109	3,132	3,558	3,302
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	399	205	33
Collectively evaluated for impairment	3,909	3,628	2,784
Acquired with deteriorated credit quality	313	316	0
Ending balance	4,621	4,149	2,817	2,019
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	1,933	1,583	1,495
Acquired with deteriorated credit quality	1,069	186	0
Ending balance	$ 3,002	$ 1,769	$ 1,495	$ 863