Allowance for loan losses and credit quality Allowance for Loan Losses, by Class of Loans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	$ 6,122	$ 5,739
Provision (credit) for loan losses	300	50
Recoveries of amounts charged off	23	4
Balance, before amounts charged off	6,445	5,793
Amounts charged off	(54)	(221)
Balance, End of Period	6,391	5,572
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	1,392	1,368
Provision (credit) for loan losses	98	37
Recoveries of amounts charged off	23	0
Balance, before amounts charged off	1,513	1,405
Amounts charged off	0	(16)
Balance, End of Period	1,513	1,389
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	774	617
Provision (credit) for loan losses	(152)	26
Recoveries of amounts charged off	0	0
Balance, before amounts charged off	622	643
Amounts charged off	0	0
Balance, End of Period	622	643
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	3,178	2,933
Provision (credit) for loan losses	335	(70)
Recoveries of amounts charged off	0	0
Balance, before amounts charged off	3,513	2,863
Amounts charged off	(54)	0
Balance, End of Period	3,459	2,863
Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	394	354
Provision (credit) for loan losses	13	177
Recoveries of amounts charged off	0	1
Balance, before amounts charged off	407	532
Amounts charged off	0	(200)
Balance, End of Period	407	332
Consumer [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	23	23
Provision (credit) for loan losses	0	2
Recoveries of amounts charged off	0	3
Balance, before amounts charged off	23	28
Amounts charged off	0	(5)
Balance, End of Period	23	23
Municipal [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	76	82
Provision (credit) for loan losses	9	10
Recoveries of amounts charged off	0	0
Balance, before amounts charged off	85	92
Amounts charged off	0	0
Balance, End of Period	85	92
Unallocated [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	285	362
Provision (credit) for loan losses	(3)	(132)
Recoveries of amounts charged off	0	0
Balance, before amounts charged off	282	230
Amounts charged off	0	0
Balance, End of Period	$ 282	$ 230