Allowance for loan losses and credit quality Allowance for Loan Losses, by Class of Loans (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				$ 5,739				$ 5,408	$ 5,739	$ 5,408
Provision (credit) for loan losses	$ 425	$ 150	$ 150	50	$ 150	$ 150	$ 150	0	775	450
Recoveries of amounts charged off									10	38
Balance, before amounts charged off									6,524	5,896
Amounts charged off									(402)	(157)
Balance, end of year	6,122				5,739				6,122	5,739
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				1,368				1,361	1,368	1,361
Provision (credit) for loan losses									150	118
Recoveries of amounts charged off									5	20
Balance, before amounts charged off									1,523	1,499
Amounts charged off									(131)	(131)
Balance, end of year	1,392				1,368				1,392	1,368
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				617				488	617	488
Provision (credit) for loan losses									157	128
Recoveries of amounts charged off									0	1
Balance, before amounts charged off									774	617
Amounts charged off									0	0
Balance, end of year	774				617				774	617
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				2,933				2,707	2,933	2,707
Provision (credit) for loan losses									305	228
Recoveries of amounts charged off									0	0
Balance, before amounts charged off									3,238	2,935
Amounts charged off									(60)	(2)
Balance, end of year	3,178				2,933				3,178	2,933
Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				354				395	354	395
Provision (credit) for loan losses									239	(38)
Recoveries of amounts charged off									1	0
Balance, before amounts charged off									594	357
Amounts charged off									(200)	(3)
Balance, end of year	394				354				394	354
Consumer [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				23				30	23	30
Provision (credit) for loan losses									7	(3)
Recoveries of amounts charged off									4	17
Balance, before amounts charged off									34	44
Amounts charged off									(11)	(21)
Balance, end of year	23				23				23	23
Municipal [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				82				64	82	64
Provision (credit) for loan losses									(6)	18
Recoveries of amounts charged off									0	0
Balance, before amounts charged off									76	82
Amounts charged off									0	0
Balance, end of year	76				82				76	82
Unallocated [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance, beginning of year				$ 362				$ 363	362	363
Provision (credit) for loan losses									(77)	(1)
Recoveries of amounts charged off									0	0
Balance, before amounts charged off									285	362
Amounts charged off									0	0
Balance, end of year	$ 285				$ 362				$ 285	$ 362