Allowance for loan losses and credit quality Allowance for Loan Losses, by Class of Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	$ 6,391	$ 5,572	$ 6,122	$ 5,739
Provision (credit) for loan losses	500	150	800	200
Recoveries of amounts charged off	0	6	23	10
Balance, before amounts charged off	6,891	5,728	6,945	5,949
Amounts charged off	(3)	(52)	(57)	(273)
Balance, End of Period	6,888	5,676	6,888	5,676
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	1,513	1,389	1,392	1,368
Provision (credit) for loan losses	77	48	175	85
Recoveries of amounts charged off	0	5	23	5
Balance, before amounts charged off	1,590	1,442	1,590	1,458
Amounts charged off	0	(46)	0	(62)
Balance, End of Period	1,590	1,396	1,590	1,396
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	622	643	774	617
Provision (credit) for loan losses	(28)	3	(180)	29
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	594	646	594	646
Amounts charged off	0	0	0	0
Balance, End of Period	594	646	594	646
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	3,459	2,863	3,178	2,933
Provision (credit) for loan losses	373	148	708	78
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	3,832	3,011	3,886	3,011
Amounts charged off	0	0	(54)	0
Balance, End of Period	3,832	3,011	3,832	3,011
Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	407	332	394	354
Provision (credit) for loan losses	84	(19)	97	158
Recoveries of amounts charged off	0	0	0	1
Balance, before amounts charged off	491	313	491	513
Amounts charged off	0	0	0	(200)
Balance, End of Period	491	313	491	313
Consumer [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	23	23	23	23
Provision (credit) for loan losses	3	5	3	7
Recoveries of amounts charged off	0	1	0	4
Balance, before amounts charged off	26	29	26	34
Amounts charged off	(3)	(6)	(3)	(11)
Balance, End of Period	23	23	23	23
Municipal [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	85	92	76	82
Provision (credit) for loan losses	(6)	(59)	3	(49)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	79	33	79	33
Amounts charged off	0	0	0	0
Balance, End of Period	79	33	79	33
Unallocated [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, Beginning of Period	282	230	285	362
Provision (credit) for loan losses	(3)	24	(6)	(108)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	279	254	279	254
Amounts charged off	0	0	0	0
Balance, End of Period	$ 279	$ 254	$ 279	$ 254