Allowance for loan losses and credit quality Impaired Loans by Class (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
With an allowance recorded, Recorded Investment		$ 796		$ 3,294	[1]	$ 2,664	[1]
With an allowance recorded, Principal Balance		837		3,322	[1]	2,682	[1]
Related Allowance		103		357		143
With no allowance recorded, Recorded Investment		4,500	[1]	1,582		2,016
With no allowance recorded, Principal Balance		4,991	[1]	1,804		2,328
Total, Recorded Investment	[1]	5,296		4,876		4,680
Total, Principal Balance	[1]	5,828		5,126		5,010
Total, Average Recorded Investment		4,968		4,750		5,108
Total, Interest Income Recognized		194		272		242
Government Guarantees on Impaired Loans		637		606		244
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an allowance recorded, Recorded Investment		308		659	[1]	537	[1]
With an allowance recorded, Principal Balance		317		668	[1]	546	[1]
Related Allowance		63		109		73
With no allowance recorded, Recorded Investment		1,140	[1]	538		413
With no allowance recorded, Principal Balance		1,561	[1]	697		602
Total, Recorded Investment	[1]	1,448		1,197		950
Total, Principal Balance	[1]	1,878		1,365		1,148
Total, Average Recorded Investment		1,303		942		805
Total, Interest Income Recognized		50		34		26
Construction Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Related Allowance		0		0		0
With no allowance recorded, Recorded Investment		88		92		275
With no allowance recorded, Principal Balance		88		92		298
Total, Recorded Investment		88		92		275
Total, Principal Balance		88		92		298
Total, Average Recorded Investment		90		162		314
Total, Interest Income Recognized		4		19		14
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With an allowance recorded, Recorded Investment		488		2,142		2,127
With an allowance recorded, Principal Balance		520		2,161		2,136
Related Allowance		40		227		70
With no allowance recorded, Recorded Investment		2,840	[1]	952		1,205
With no allowance recorded, Principal Balance		2,910	[1]	1,015		1,256
Total, Recorded Investment		3,328	[1]	3,094		3,332
Total, Principal Balance		3,430	[1]	3,176		3,392
Total, Average Recorded Investment		3,113		3,523		3,883
Total, Interest Income Recognized		107		219		195
Commercial [Member]
Financing Receivable, Impaired [Line Items]
With an allowance recorded, Recorded Investment	[1]			493
With an allowance recorded, Principal Balance	[1]			493
Related Allowance		0		21		0
With no allowance recorded, Recorded Investment		432	[1]			123
With no allowance recorded, Principal Balance		432	[1]			172
Total, Recorded Investment		432	[1]	493	[1]	123
Total, Principal Balance		432	[1]	493	[1]	172
Total, Average Recorded Investment		462		123		106
Total, Interest Income Recognized		$ 33		$ 0		$ 7

[1] Does not reflect government guaranties on impaired loans as of as of December 31, 2016, 2015 and 2014 totaling $637 thousand, $606 thousand and $244 thousand, respectively.