Loans and allowance for credit losses - Individually Assessed Allowance (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 113,129	$ 89,141	$ 31,139	$ 31,139	$ 30,138	$ 29,814	$ 28,932
Individually assessed allowance for credit loss	2,185
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	8,878	10,881		4,805	4,923	5,514	5,348
Individually assessed allowance for credit loss	543
Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	35,599	22,842		10,194	9,655	9,758	9,729
Individually assessed allowance for credit loss	0
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	12,463	13,006		3,112	3,288	3,295	3,428
Individually assessed allowance for credit loss	0
Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	6,811	6,213		752	755	731	811
Individually assessed allowance for credit loss	9
Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	4,499	2,328		544	617	539	566
Individually assessed allowance for credit loss	0
Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	7,420	9,047		4,109	3,512	3,098	3,132
Individually assessed allowance for credit loss	35
Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	30,444	18,005		4,621	4,478	4,583	4,149
Individually assessed allowance for credit loss	1,563
Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	7,015	$ 6,819		$ 3,002	$ 2,910	$ 2,296	$ 1,769
Individually assessed allowance for credit loss	35
Real Estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	11,764
Real Estate | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Real Estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,232
Real Estate | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	100
Real Estate | Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	471
Real Estate | Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Real Estate | Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	1,461
Real Estate | Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	8,500
Real Estate | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	3,398
Financial Assets and Equipment | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	3,062
Financial Assets and Equipment | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Residential real estate: | Residential line of credit
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Residential real estate: | Multi-family mortgage
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Commercial real estate: | Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Commercial real estate: | Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	0
Financial Assets and Equipment | Consumer and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans	$ 336