Allowance for loan losses and credit quality Allocation of the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	$ 225		$ 66
Collectively evaluated for impairment	5,583		5,673
Total allocated	5,808	$ 5,676	5,739	$ 5,610	$ 5,553	$ 5,408
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	41		47
Collectively evaluated for impairment	1,368		1,321
Total allocated	1,409	1,396	1,368	1,408	1,375	1,361
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	689		617
Total allocated	689	646	617	602	556	488
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	176		9
Collectively evaluated for impairment	2,991		2,924
Total allocated	3,167	3,011	2,933	2,876	2,855	2,707
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	8		10
Collectively evaluated for impairment	318		344
Total allocated	326	313	354	364	374	395
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	25		23
Total allocated	25	23	23	23	26	30
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	78		82
Total allocated	78	33	82	81	30	64
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	114		362
Total allocated	$ 114	$ 254	$ 362	$ 256	$ 337	$ 363