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	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 735		$ 268
Collectively evaluated for impairment	29,167		27,783
Acquired with deteriorated credit quality	1,562		881
Ending balance	31,464	$ 30,138	28,932	$ 27,608	$ 26,347	$ 24,041
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	13		3
Collectively evaluated for impairment	5,064		5,247
Acquired with deteriorated credit quality	93		98
Ending balance	5,170	4,923	5,348	5,365	4,747	4,461
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	9,796		9,677
Acquired with deteriorated credit quality	46		52
Ending balance	9,842	9,655	9,729	8,268	9,023	7,135
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	11		7
Collectively evaluated for impairment	3,173		3,205
Acquired with deteriorated credit quality	147		216
Ending balance	3,331	3,288	3,428	3,520	3,378	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	727		811
Acquired with deteriorated credit quality	0		0
Ending balance	727	755	811	838	795	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	574		566
Acquired with deteriorated credit quality	0		0
Ending balance	574	617	566	683	391	434
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	52		53
Collectively evaluated for impairment	3,641		3,066
Acquired with deteriorated credit quality	16		13
Ending balance	3,709	3,512	3,132	3,481	3,290	3,558
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	405		205
Collectively evaluated for impairment	4,225		3,628
Acquired with deteriorated credit quality	297		316
Ending balance	4,927	4,478	4,149	3,911	3,272	2,817
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	254		0
Collectively evaluated for impairment	1,967		1,583
Acquired with deteriorated credit quality	963		186
Ending balance	$ 3,184	$ 2,910	$ 1,769	$ 1,542	$ 1,451	$ 1,495