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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				$ 4,694				$ 4,647				$ 4,657	$ 4,694	$ 4,647	$ 4,657
Provision (credit) for loan loss	$ 150	$ 150	$ 150	100	$ 45	$ 150	$ 75	75	$ 75	$ 95	$ 75	60	550	345	305
Recoveries of amounts charged off													83	42	87
Balance, before amounts charged off	5,327				5,034				5,049				5,327	5,034	5,049
Amounts charged off													(126)	(340)	(402)
Balance, end of year	5,201				4,694				4,647				5,201	4,694	4,647
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				1,330				1,251				1,291	1,330	1,251	1,291
Provision (credit) for loan loss													136	177	293
Recoveries of amounts charged off													36	1	13
Balance, before amounts charged off	1,502				1,429				1,597				1,502	1,429	1,597
Amounts charged off													(83)	(99)	(346)
Balance, end of year	1,419				1,330				1,251				1,419	1,330	1,251
Construction Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				439				390				456	439	390	456
Provision (credit) for loan loss													47	37	(62)
Recoveries of amounts charged off													28	12	12
Balance, before amounts charged off	514				439				406				514	439	406
Amounts charged off													0	0	(16)
Balance, end of year	514				439				390				514	439	390
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				2,417				2,644				2,532	2,417	2,644	2,532
Provision (credit) for loan loss													375	(93)	77
Recoveries of amounts charged off													0	8	35
Balance, before amounts charged off	2,792				2,559				2,644				2,792	2,559	2,644
Amounts charged off													0	(142)	0
Balance, end of year	2,792				2,417				2,644				2,792	2,417	2,644
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				176				163				159	176	163	159
Provision (credit) for loan loss													46	59	22
Recoveries of amounts charged off													16	8	6
Balance, before amounts charged off	238				230				187				238	230	187
Amounts charged off													(29)	(54)	(24)
Balance, end of year	209				176				163				209	176	163
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				27				23				39	27	23	39
Provision (credit) for loan loss													12	36	(21)
Recoveries of amounts charged off													3	13	21
Balance, before amounts charged off	42				72				39				42	72	39
Amounts charged off													(14)	(45)	(16)
Balance, end of year	28				27				23				28	27	23
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				42				35				30	42	35	30
Provision (credit) for loan loss													(4)	7	5
Recoveries of amounts charged off													0	0	0
Balance, before amounts charged off	38				42				35				38	42	35
Amounts charged off													0	0	0
Balance, end of year	38				42				35				38	42	35
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year				$ 263				$ 141				$ 150	263	141	150
Provision (credit) for loan loss													(62)	122	(9)
Recoveries of amounts charged off													0	0	0
Balance, before amounts charged off	201				263				141				201	263	141
Amounts charged off													0	0	0
Balance, end of year	$ 201				$ 263				$ 141				$ 201	$ 263	$ 141