Allowance for loan losses and credit quality Impaired Loans by Class (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016		Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	$ 696				$ 696				$ 796
With an allowance recorded, Principal Balance	705				705				837
Related Allowance	83		$ 142		83		$ 142		103
With no allowance recorded, Recorded Investment	3,335	[1]			3,335	[1]			4,500	[2]
With no allowance recorded, Principal Balance	3,917	[1]			3,917	[1]			4,991	[2]
Total, Recorded Investment	4,031	[1]	5,007	[3]	4,031	[1]	5,007	[3]	5,296	[2]
Total, Principal Balance	4,622	[1]	5,461	[3]	4,622	[1]	5,461	[3]	5,828	[2]
Total, Average Recorded Investment	4,167		4,928		4,543		4,911
Total, Interest Income Recognized	48		26		99		49
Government Guarantees on Impaired Loans	635		578		635		578		637
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	303				303				308
With an allowance recorded, Principal Balance	312				312				317
Related Allowance	59		58		59		58		63
With no allowance recorded, Recorded Investment	1,467	[1]			1,467	[1]			1,140	[2]
With no allowance recorded, Principal Balance	1,977	[1]			1,977	[1]			1,561	[2]
Total, Recorded Investment	1,770	[1]	1,304	[3]	1,770	[1]	1,304	[3]	1,448	[2]
Total, Principal Balance	2,289	[1]	1,663	[3]	2,289	[1]	1,663	[3]	1,878	[2]
Total, Average Recorded Investment	1,675		1,240		1,599		1,226
Total, Interest Income Recognized	20		8		31		16
Construction Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Related Allowance	0		0		0		0		0
With no allowance recorded, Recorded Investment	85				85				88
With no allowance recorded, Principal Balance	85				85				88
Total, Recorded Investment	85		90		85		90		88
Total, Principal Balance	85		90		85		90		88
Total, Average Recorded Investment	86		90		86		91
Total, Interest Income Recognized	1		1		2		2
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	393				393				488
With an allowance recorded, Principal Balance	393				393				520
Related Allowance	24		84		24		84		40
With no allowance recorded, Recorded Investment	1,375	[1]			1,375	[1]			2,840	[2]
With no allowance recorded, Principal Balance	1,447	[1]			1,447	[1]			2,910	[2]
Total, Recorded Investment	1,768	[1]	3,153		1,768	[1]	3,153		3,328	[2]
Total, Principal Balance	1,840	[1]	3,247		1,840	[1]	3,247		3,430	[2]
Total, Average Recorded Investment	1,994		3,130		2,439		3,118
Total, Interest Income Recognized	22		17		54		31
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Related Allowance	0		0		0		0		0
With no allowance recorded, Recorded Investment	408	[1]			408	[1]			432	[2]
With no allowance recorded, Principal Balance	408	[1]			408	[1]			432	[2]
Total, Recorded Investment	408	[1]	460	[3]	408	[1]	460	[3]	432	[2]
Total, Principal Balance	408	[1]	461	[3]	408	[1]	461	[3]	$ 432	[2]
Total, Average Recorded Investment	412		468		419		476
Total, Interest Income Recognized	$ 5		$ 0		$ 12		$ 0

[1] Does not reflect government guaranties on impaired loans as of June 30, 2017 totaling $635 thousand.

[2] Does not reflect government guaranties on impaired loans as of December 31, 2016 totaling $637 thousand.

[3] Does not reflect government guaranties on impaired loans as of June 30, 2016 totaling $578 thousand.