Allowance for loan losses and credit quality Impaired Loans by Class (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016		Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	$ 440				$ 440				$ 796
With an allowance recorded, Principal Balance	453				453				837
Related Allowance	61		$ 118		61		$ 118		103
With no allowance recorded, Recorded Investment	3,460	[1]			3,460	[1]			4,500	[2]
With no allowance recorded, Principal Balance	4,062	[1]			4,062	[1]			4,991	[2]
Total, Recorded Investment	3,900	[1]	4,811	[3]	3,900	[1]	4,811	[3]	5,296	[2]
Total, Principal Balance	4,515	[1]	5,309	[3]	4,515	[1]	5,309	[3]	5,828	[2]
Total, Average Recorded Investment	3,965		4,909		4,382		4,886
Total, Interest Income Recognized	49		36		148		85
Government Guarantees on Impaired Loans	564		654		564		654		637
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	300				300				308
With an allowance recorded, Principal Balance	310				310				317
Related Allowance	57		57		57		57		63
With no allowance recorded, Recorded Investment	1,639	[1]			1,639	[1]			1,140	[2]
With no allowance recorded, Principal Balance	2,168	[1]			2,168	[1]			1,561	[2]
Total, Recorded Investment	1,939	[1]	1,388	[3]	1,939	[1]	1,388	[3]	1,448	[2]
Total, Principal Balance	2,478	[1]	1,788	[3]	2,478	[1]	1,788	[3]	1,878	[2]
Total, Average Recorded Investment	1,855		1,346		1,684		1,266
Total, Interest Income Recognized	23		7		54		23
Construction Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Related Allowance	0		0		0		0		0
With no allowance recorded, Recorded Investment	84				84				88
With no allowance recorded, Principal Balance	84				84				88
Total, Recorded Investment	84		89		84		89		88
Total, Principal Balance	84		89		84		89		88
Total, Average Recorded Investment	84		89		86		91
Total, Interest Income Recognized	1		1		3		3
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an alowance recorded, Recorded Investment	140				140				488
With an allowance recorded, Principal Balance	143				143				520
Related Allowance	4		61		4		61		40
With no allowance recorded, Recorded Investment	1,344	[1]			1,344	[1]			2,840	[2]
With no allowance recorded, Principal Balance	1,417	[1]			1,417	[1]			2,910	[2]
Total, Recorded Investment	1,484	[1]	2,883	[3]	1,484	[1]	2,883	[3]	3,328	[2]
Total, Principal Balance	1,560	[1]	2,981	[3]	1,560	[1]	2,981	[3]	3,430	[2]
Total, Average Recorded Investment	1,626		3,018		2,200		3,059
Total, Interest Income Recognized	18		28		72		59
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Related Allowance	0		0		0		0		0
With no allowance recorded, Recorded Investment	393	[1]			393	[1]			432	[2]
With no allowance recorded, Principal Balance	393	[1]			393	[1]			432	[2]
Total, Recorded Investment	393	[1]	451	[3]	393	[1]	451	[3]	432	[2]
Total, Principal Balance	393	[1]	451	[3]	393	[1]	451	[3]	$ 432	[2]
Total, Average Recorded Investment	400		456		412		470
Total, Interest Income Recognized	$ 7		$ 0		$ 19		$ 0

[1] Does not reflect government guaranties on impaired loans as of September 30, 2017 totaling $564 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 September 30, 2016 totaling $654 thousand.