Fair value estimates - Carrying and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment securities:
Available-for-sale, Fair Value	$ 249,913	$ 284,935
Held-to-maturity, Fair Value	11,546	12,606
Mortgage servicing rights	681	465
Carrying Value
Financial assets:
Cash and cash equivalents	31,914	32,917
Investment securities:
Available-for-sale, Fair Value	249,913	284,935
Held-to-maturity, Fair Value	10,864	12,109
Loans held for sale	360
Net loans	1,506,679	1,327,890
Accrued interest receivable	5,309	5,796
Mortgage servicing rights	681	465
Restricted equity securities	6,911	5,403
Assets, Fair Value Disclosure	1,812,631	1,669,515
Financial liabilities:
Deposits	1,565,810	1,455,810
Short-term borrowings	75,300	38,325
Long-term debt	58,685	60,354
Accrued interest payable	434	560
Liabilities, total	1,700,229	1,555,049
Fair Value
Financial assets:
Cash and cash equivalents	31,914	32,917
Investment securities:
Available-for-sale, Fair Value	249,913	284,935
Held-to-maturity, Fair Value	11,546	12,606
Loans held for sale	359
Net loans	1,517,858	1,330,900
Accrued interest receivable	5,309	5,796
Mortgage servicing rights	1,551	1,543
Restricted equity securities	6,911	5,403
Assets, Fair Value Disclosure	1,825,361	1,674,100
Financial liabilities:
Deposits	1,567,788	1,455,979
Short-term borrowings	75,300	38,325
Long-term debt	60,560	61,412
Accrued interest payable	434	560
Liabilities, total	1,704,082	1,556,276
Level 1
Financial assets:
Cash and cash equivalents	31,914	32,917
Investment securities:
Available-for-sale, Fair Value		9,999
Level 2
Investment securities:
Available-for-sale, Fair Value	249,913	274,936
Held-to-maturity, Fair Value	11,546	12,606
Loans held for sale	359
Accrued interest receivable	5,309	5,796
Mortgage servicing rights	1,551	1,543
Restricted equity securities	6,911	5,403
Financial liabilities:
Deposits	1,567,788	1,455,979
Short-term borrowings	75,300	38,325
Long-term debt	60,560	61,412
Accrued interest payable	434	560
Level 3
Investment securities:
Net loans	$ 1,517,858	$ 1,330,900