Allowance for loan losses and credit quality Loan Ratings by Class (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 578,902	$ 533,290
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	462,943	443,132
Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	108,107	80,807
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	7,852	9,351
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	176,399	172,727
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	163,064	158,140
Residential Real Estate [Member] | Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	9,921	10,641
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3,414	3,946
Construction Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	36,796	34,189
Construction Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	29,466	29,248
Construction Real Estate [Member] | Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	7,164	4,830
Construction Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	166	111
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	257,192	249,063
Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	166,776	182,247
Commercial Real Estate [Member] | Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	86,809	62,193
Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3,607	4,623
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	48,166	41,999
Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	43,323	38,219
Commercial [Member] | Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	4,181	3,109
Commercial [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	662	671
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3,832	3,962
Consumer [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3,797	3,928
Consumer [Member] | Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	32	34
Consumer [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	3	0
Municipal [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	56,517	31,350
Municipal [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	56,517	31,350
Municipal [Member] | Satisfactory/Monitor [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Municipal [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 0	$ 0