Allowance for loan losses and credit quality Allocation of the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	$ 130		$ 196
Collectively evaluated for impairment	6,758		5,926
Total allocated	6,888	$ 6,391	6,122	$ 5,676	$ 5,572	$ 5,739
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	35		39
Collectively evaluated for impairment	1,555		1,353
Total allocated	1,590	1,513	1,392	1,396	1,389	1,368
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	2		0
Collectively evaluated for impairment	592		774
Total allocated	594	622	774	646	643	617
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	86		149
Collectively evaluated for impairment	3,746		3,029
Total allocated	3,832	3,459	3,178	3,011	2,863	2,933
Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	7		8
Collectively evaluated for impairment	484		386
Total allocated	491	407	394	313	332	354
Consumer [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	23		23
Total allocated	23	23	23	23	23	23
Municipal [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	79		76
Total allocated	79	85	76	33	92	82
Unallocated [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	279		285
Total allocated	$ 279	$ 282	$ 285	$ 254	$ 230	$ 362