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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				$ 5,201				$ 4,694				$ 4,647	$ 5,201	$ 4,694	$ 4,647
Provision (credit) for loan losses	$ 0	$ 0	$ 75	75	$ 150	$ 150	$ 150	100	$ 45	$ 150	$ 75	75	150	550	345
Recoveries of amounts charged off													59	83	42
Balance, before amounts charged off	5,410				5,327				5,034				5,410	5,327	5,034
Amounts charged off													(163)	(126)	(340)
Balance, end of year	5,247				5,201				4,694				5,247	5,201	4,694
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				1,419				1,330				1,251	1,419	1,330	1,251
Provision (credit) for loan losses													64	136	177
Recoveries of amounts charged off													36	36	1
Balance, before amounts charged off	1,519				1,502				1,429				1,519	1,502	1,429
Amounts charged off													(120)	(83)	(99)
Balance, end of year	1,399				1,419				1,330				1,399	1,419	1,330
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				514				439				390	514	439	390
Provision (credit) for loan losses													(135)	47	37
Recoveries of amounts charged off													12	28	12
Balance, before amounts charged off	391				514				439				391	514	439
Amounts charged off													0	0	0
Balance, end of year	391				514				439				391	514	439
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				2,792				2,417				2,644	2,792	2,417	2,644
Provision (credit) for loan losses													(105)	375	(93)
Recoveries of amounts charged off													0	0	8
Balance, before amounts charged off	2,687				2,792				2,559				2,687	2,792	2,559
Amounts charged off													0	0	(142)
Balance, end of year	2,687				2,792				2,417				2,687	2,792	2,417
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				209				176				163	209	176	163
Provision (credit) for loan losses													158	46	59
Recoveries of amounts charged off													8	16	8
Balance, before amounts charged off	375				238				230				375	238	230
Amounts charged off													(33)	(29)	(54)
Balance, end of year	342				209				176				342	209	176
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				28				27				23	28	27	23
Provision (credit) for loan losses													5	12	36
Recoveries of amounts charged off													3	3	13
Balance, before amounts charged off	36				42				72				36	42	72
Amounts charged off													(10)	(14)	(45)
Balance, end of year	26				28				27				26	28	27
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				38				42				35	38	42	35
Provision (credit) for loan losses													2	(4)	7
Recoveries of amounts charged off													0	0	0
Balance, before amounts charged off	40				38				42				40	38	42
Amounts charged off													0	0	0
Balance, end of year	40				38				42				40	38	42
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				$ 201				$ 263				$ 141	201	263	141
Provision (credit) for loan losses													161	(62)	122
Recoveries of amounts charged off													0	0	0
Balance, before amounts charged off	362				201				263				362	201	263
Amounts charged off													0	0	0
Balance, end of year	$ 362				$ 201				$ 263				$ 362	$ 201	$ 263