Allowance for loan losses and credit quality Impaired Loans by Class (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019		Dec. 31, 2019
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	$ 1,987	[1]			$ 1,991
With an allowance recorded, Principal Balance	2,034	[1]			2,023
Related Allowance	208		$ 67		196
With no allowance recorded, Recorded Investment	3,157	[1]			3,250	[2]
With no allowance recorded, Principal Balance	3,821	[1]			3,902	[2]
Total, Recorded Investment	5,144	[1]	3,843	[3]	5,241	[2]
Total, Principal Balance	5,855	[1]	4,578	[3]	5,925	[2]
Total, Average Recorded Investment	5,193		4,134
Total, Interest Income Recognized	49		65
Government Guarantees on Impaired Loans	570		630		587
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	216				218
With an allowance recorded, Principal Balance	226				228
Related Allowance	68		46		39
With no allowance recorded, Recorded Investment	1,272	[1]			1,297	[2]
With no allowance recorded, Principal Balance	1,818	[1]			1,832	[2]
Total, Recorded Investment	1,488	[1]	1,720	[3]	1,515	[2]
Total, Principal Balance	2,044	[1]	2,344	[3]	2,060	[2]
Total, Average Recorded Investment	1,502		1,699
Total, Interest Income Recognized	19		19
Construction Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Related Allowance	0		0		0
With no allowance recorded, Recorded Investment	218				223
With no allowance recorded, Principal Balance	237				241
Total, Recorded Investment	218		114		223
Total, Principal Balance	237		131		241
Total, Average Recorded Investment	220		116
Total, Interest Income Recognized	1		1
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	1,746				1,762
With an allowance recorded, Principal Balance	1,781				1,783
Related Allowance	133		11		149
With no allowance recorded, Recorded Investment	1,412	[1]			1,442	[2]
With no allowance recorded, Principal Balance	1,509	[1]			1,539	[2]
Total, Recorded Investment	3,158	[1]	1,669	[3]	3,204	[2]
Total, Principal Balance	3,290	[1]	1,761	[3]	3,322	[2]
Total, Average Recorded Investment	3,181		1,973
Total, Interest Income Recognized	22		40
Commercial [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	25	[1]			11
With an allowance recorded, Principal Balance	27	[1]			12
Related Allowance	7		10		8
With no allowance recorded, Recorded Investment	255	[1]			288	[2]
With no allowance recorded, Principal Balance	257	[1]			290	[2]
Total, Recorded Investment	280	[1]	340	[3]	299	[2]
Total, Principal Balance	284	[1]	342	[3]	$ 302	[2]
Total, Average Recorded Investment	290		346
Total, Interest Income Recognized	$ 7		$ 5

[1] Does not reflect government guaranties on impaired loans as of March 31, 2020 totaling $570 thousand.

[2] Does not reflect government guaranties on impaired loans as of December 31, 2019 totaling $587 thousand.

[3] Does not reflect government guaranties on impaired loans as of March 31, 2019 totaling $630 thousand.