Fair value estimates - Carrying and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment securities:
Available-for-sale, Fair Value	$ 264,644	$ 259,410
Held-to-maturity, Fair value	10,359	10,714
Mortgage servicing rights	698	698
Carrying Value
Financial assets:
Cash and cash equivalents	31,815	39,941
Investment securities:
Available-for-sale, Fair Value	264,644	259,410
Held-to-maturity, Fair value	10,180	10,517
Loans held for sale	444
Net loans	1,542,898	1,517,004
Accrued interest receivable	5,604	6,228
Mortgage servicing rights	698	698
Restricted equity securities	6,781	7,051
Assets, Fair Value Disclosure	1,863,064	1,840,849
Financial liabilities:
Deposits	1,615,544	1,588,757
Short-term borrowings	77,475	82,700
Long-term debt	57,615	58,134
Accrued interest payable	457	462
Liabilities, total	1,751,091	1,730,053
Fair Value
Financial assets:
Cash and cash equivalents	31,815	39,941
Investment securities:
Available-for-sale, Fair Value	264,644	259,410
Held-to-maturity, Fair value	10,359	10,714
Loans held for sale	453
Net loans	1,529,464	1,507,936
Accrued interest receivable	5,604	6,228
Mortgage servicing rights	1,587	1,587
Restricted equity securities	6,781	7,051
Assets, Fair Value Disclosure	1,850,707	1,832,867
Financial liabilities:
Deposits	1,614,470	1,587,701
Short-term borrowings	77,475	82,700
Long-term debt	58,484	58,987
Accrued interest payable	457	462
Liabilities, total	1,750,886	1,729,850
Level 1
Financial assets:
Cash and cash equivalents	31,815	39,941
Investment securities:
Available-for-sale, Fair Value	19,895	7,438
Level 2
Investment securities:
Available-for-sale, Fair Value	244,749	251,972
Held-to-maturity, Fair value	10,359	10,714
Loans held for sale	453
Accrued interest receivable	5,604	6,228
Mortgage servicing rights	1,587	1,587
Restricted equity securities	6,781	7,051
Financial liabilities:
Deposits	1,614,470	1,587,701
Short-term borrowings	77,475	82,700
Long-term debt	58,484	58,987
Accrued interest payable	457	462
Level 3
Investment securities:
Net loans	$ 1,529,464	$ 1,507,936