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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	$ 5,676	$ 5,553	$ 5,739	$ 5,408
Provision (credit) for loan losses	150	150	350	300
Recoveries of amounts charged off	0	13	10	17
Balance, before amounts charged off	5,826	5,716	6,099	5,725
Amounts charged off	(18)	(106)	(291)	(115)
Balance, End of Period	5,808	5,610	5,808	5,610
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	1,396	1,375	1,368	1,361
Provision (credit) for loan losses	31	133	116	147
Recoveries of amounts charged off	0	0	5	0
Balance, before amounts charged off	1,427	1,508	1,489	1,508
Amounts charged off	(18)	(100)	(80)	(100)
Balance, End of Period	1,409	1,408	1,409	1,408
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	646	556	617	488
Provision (credit) for loan losses	43	46	72	114
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	689	602	689	602
Amounts charged off	0	0	0	0
Balance, End of Period	689	602	689	602
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	3,011	2,855	2,933	2,707
Provision (credit) for loan losses	156	21	234	171
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	3,167	2,876	3,167	2,878
Amounts charged off	0	0	0	(2)
Balance, End of Period	3,167	2,876	3,167	2,876
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	313	374	354	395
Provision (credit) for loan losses	13	(10)	171	(31)
Recoveries of amounts charged off	0	0	1	0
Balance, before amounts charged off	326	364	526	364
Amounts charged off	0	0	(200)	0
Balance, End of Period	326	364	326	364
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	23	26	23	30
Provision (credit) for loan losses	2	(10)	9	(11)
Recoveries of amounts charged off	0	13	4	17
Balance, before amounts charged off	25	29	36	36
Amounts charged off	0	(6)	(11)	(13)
Balance, End of Period	25	23	25	23
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	33	30	82	64
Provision (credit) for loan losses	45	51	(4)	17
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	78	81	78	81
Amounts charged off	0	0	0	0
Balance, End of Period	78	81	78	81
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, Beginning of Period	254	337	362	363
Provision (credit) for loan losses	(140)	(81)	(248)	(107)
Recoveries of amounts charged off	0	0	0	0
Balance, before amounts charged off	114	256	114	256
Amounts charged off	0	0	0	0
Balance, End of Period	$ 114	$ 256	$ 114	$ 256