Loans and allowance for credit 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, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				$ 895
Collectively evaluated for impairment				28,571
Acquired with deteriorated credit quality				1,673
Ending balance	$ 183,973	$ 113,129	$ 31,139	31,139	$ 31,464	$ 30,138	$ 28,932
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				241
Collectively evaluated for impairment				4,457
Acquired with deteriorated credit quality				107
Ending balance	8,609	8,878		4,805	5,170	4,923	5,348
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				0
Collectively evaluated for impairment				10,192
Acquired with deteriorated credit quality				2
Ending balance	63,629	35,599		10,194	9,842	9,655	9,729
Residential real estate: | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				8
Collectively evaluated for impairment				2,940
Acquired with deteriorated credit quality				164
Ending balance	18,298	12,463		3,112	3,331	3,288	3,428
Residential real estate: | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				9
Collectively evaluated for impairment				743
Acquired with deteriorated credit quality				0
Ending balance	11,455	6,811		752	727	755	811
Residential real estate: | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				0
Collectively evaluated for impairment				544
Acquired with deteriorated credit quality				0
Ending balance	7,191	4,499		544	574	617	566
Commercial real estate: | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				238
Collectively evaluated for impairment				3,853
Acquired with deteriorated credit quality				18
Ending balance	6,705	7,420		4,109	3,709	3,512	3,132
Commercial real estate: | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				399
Collectively evaluated for impairment				3,909
Acquired with deteriorated credit quality				313
Ending balance	57,640	30,444		4,621	4,927	4,478	4,149
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				0
Collectively evaluated for impairment				1,933
Acquired with deteriorated credit quality				1,069
Ending balance	$ 10,446	$ 7,015		$ 3,002	$ 3,184	$ 2,910	$ 1,769