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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 1,186	$ 268
Collectively evaluated for impairment	27,363	27,783
Acquired with deteriorated credit quality	1,265	881
Ending balance	29,814	28,932	$ 24,406	$ 24,041
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	709	3
Collectively evaluated for impairment	4,697	5,247
Acquired with deteriorated credit quality	108	98
Ending balance	5,514	5,348	4,578	4,461
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	9,711	9,677
Acquired with deteriorated credit quality	47	52
Ending balance	9,758	9,729	7,866	7,135
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	136	7
Collectively evaluated for impairment	3,027	3,205
Acquired with deteriorated credit quality	132	216
Ending balance	3,295	3,428	3,122	3,197
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	731	811
Acquired with deteriorated credit quality	0	0
Ending balance	731	811	1,165	944
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	539	566
Acquired with deteriorated credit quality	0	0
Ending balance	539	566	449	434
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	42	53
Collectively evaluated for impairment	3,039	3,066
Acquired with deteriorated credit quality	17	13
Ending balance	3,098	3,132	3,014	3,558
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	299	205
Collectively evaluated for impairment	4,068	3,628
Acquired with deteriorated credit quality	216	316
Ending balance	4,583	4,149	2,753	2,817
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	1,551	1,583
Acquired with deteriorated credit quality	745	186
Ending balance	$ 2,296	$ 1,769	$ 1,459	$ 1,495