Allowance for loan losses and credit quality Allowance for Loan Losses, by Class of Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	$ 5,192	$ 5,125	$ 5,247	$ 5,201
Provision (credit) for loan losses	0	75	0	150
Recoveries of amounts charged off	4	26	10	31
Balance, before amounts charged off	5,196	5,226	5,257	5,382
Amounts charged off	(28)	0	(89)	(156)
Balance, End of Period	5,168	5,226	5,168	5,226
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	1,372	1,386	1,399	1,419
Provision (credit) for loan losses	39	(19)	68	68
Recoveries of amounts charged off	0	15	2	15
Balance, before amounts charged off	1,411	1,382	1,469	1,502
Amounts charged off	(24)	0	(82)	(120)
Balance, End of Period	1,387	1,382	1,387	1,382
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	442	466	391	514
Provision (credit) for loan losses	36	(96)	84	(147)
Recoveries of amounts charged off	3	3	6	6
Balance, before amounts charged off	481	373	481	373
Amounts charged off	0	0	0	0
Balance, End of Period	481	373	481	373
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	2,661	2,729	2,687	2,792
Provision (credit) for loan losses	92	108	66	45
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	2,753	2,837	2,753	2,837
Amounts charged off	0	0	0	0
Balance, End of Period	2,753	2,837	2,753	2,837
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	346	218	342	209
Provision (credit) for loan losses	15	16	19	57
Recoveries of amounts charged off	1	6	1	7
Balance, before amounts charged off	362	240	362	273
Amounts charged off	0	0	0	(33)
Balance, End of Period	362	240	362	240
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	24	25	26	28
Provision (credit) for loan losses	4	0	4	(1)
Recoveries of amounts charged off	0	2	1	3
Balance, before amounts charged off	28	27	31	30
Amounts charged off	(4)	0	(7)	(3)
Balance, End of Period	24	27	24	27
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	42	47	40	38
Provision (credit) for loan losses	(16)	(21)	(14)	(12)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	26	26	26	26
Amounts charged off	0	0	0	0
Balance, End of Period	26	26	26	26
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	305	254	362	201
Provision (credit) for loan losses	(170)	87	(227)	140
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	135	341	135	341
Amounts charged off	0	0	0	0
Balance, End of Period	$ 135	$ 341	$ 135	$ 341