Summary of significant accounting policies - Recent accounting standards (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) loan	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) loan customer	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) loan
Interest and Fee Income, Loans and Leases, Total	$ 20,029,000	$ 21,160,000	$ 40,929,000	$ 42,077,000
Number of business loans with payment deferrals granted | customer			4
Outstanding loan balance in deferral	$ 69,000		$ 69,000
PPP | SBA PPP
Number of loans originated | loan	1,062		1,062		1,450
Principal amount of loan originated	$ 121,600,000		$ 121,600,000		$ 217,500,000
Average loan size					150,000
Net fees on loans					5,200,000
Fees income			4,400,000
Amount of loan forgiven	$ 156,900,000		156,900,000		27,800,000
Interest and Fee Income, Loans and Leases, Total			$ 2,800,000		$ 2,300,000
PPP | Minimum | SBA PPP
Amortization period					24 months
PPP | Maximum | SBA PPP
Amortization period					60 months
Commercial loans
Number of business loans with payment deferrals granted | customer			4
Commercial loans | SBA PPP
Number of business loans with payment deferrals granted | loan					479
Loan receivable balances where payment deferrals were granted					$ 306,800,000
Consumer Loan | SBA PPP
Number of loans modified | loan	4		4
Amount of loans modified	$ 68,000		$ 68,000
Number of non-performing loans | loan	2		2
Outstanding balance of non-performing loans			$ 32,000
Number of business loans with payment deferrals granted | loan					512
Loan receivable balances where payment deferrals were granted					$ 23,300,000