Allowance for loan losses and credit quality Allowance for Loan Losses, by Class of Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	$ 5,168	$ 5,226	$ 5,247	$ 5,201
Provision (credit) for loan losses	150	0	150	150
Recoveries of amounts charged off	43	4	53	35
Balance, before amounts charged off	5,361	5,230	5,450	5,386
Amounts charged off	(102)	(4)	(191)	(160)
Balance, End of Period	5,259	5,226	5,259	5,226
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	1,387	1,382	1,399	1,419
Provision (credit) for loan losses	56	11	124	79
Recoveries of amounts charged off	36	0	38	15
Balance, before amounts charged off	1,479	1,393	1,561	1,513
Amounts charged off	(100)	0	(182)	(120)
Balance, End of Period	1,379	1,393	1,379	1,393
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	481	373	391	514
Provision (credit) for loan losses	(76)	28	8	(119)
Recoveries of amounts charged off	3	3	9	9
Balance, before amounts charged off	408	404	408	404
Amounts charged off	0	0	0	0
Balance, End of Period	408	404	408	404
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	2,753	2,837	2,687	2,792
Provision (credit) for loan losses	37	(64)	103	(19)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	2,790	2,773	2,790	2,773
Amounts charged off	0	0	0	0
Balance, End of Period	2,790	2,773	2,790	2,773
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	362	240	342	209
Provision (credit) for loan losses	(6)	5	13	62
Recoveries of amounts charged off	3	1	4	8
Balance, before amounts charged off	359	246	359	279
Amounts charged off	0	0	0	(33)
Balance, End of Period	359	246	359	246
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	24	27	26	28
Provision (credit) for loan losses	3	4	7	3
Recoveries of amounts charged off	1	0	2	3
Balance, before amounts charged off	28	31	35	34
Amounts charged off	(2)	(4)	(9)	(7)
Balance, End of Period	26	27	26	27
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	26	26	40	38
Provision (credit) for loan losses	39	20	25	8
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	65	46	65	46
Amounts charged off	0	0	0	0
Balance, End of Period	65	46	65	46
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	135	341	362	201
Provision (credit) for loan losses	97	(4)	(130)	136
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	232	337	232	337
Amounts charged off	0	0	0	0
Balance, End of Period	$ 232	$ 337	$ 232	$ 337