Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial assets:
Available-for-sale debt securities, at fair value	$ 668,531	$ 655,698
Equity securities, at fair value	3,250	3,107
Impaired loans	18,267	13,716
Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	173,785	138,887
Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,432	7,242
Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	1,015
Quoted prices in active markets for identical assets (liabilities) (level 1)
Financial assets:
Investment securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Significant other observable inputs (level 2)
Financial assets:
Investment securities	671,781	658,805
Loans held for sale	305,493	278,815
Mortgage servicing rights	0	0
Derivatives	29,172	14,316
Financial liabilities:
Derivatives	19,887	11,637
Significant unobservable inputs (level 3)
Financial assets:
Investment securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	66,156	88,829
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring
Financial assets:
Equity securities, at fair value	3,250	3,107
Investment securities	671,781	658,805
Loans held for sale	305,493	278,815
Mortgage servicing rights	66,156	88,829
Derivatives	29,172	14,316
Financial liabilities:
Derivatives	19,887	11,637
Fair Value, Measurements, Recurring | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	999	989
Fair Value, Measurements, Recurring | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	485,300	508,580
Fair Value, Measurements, Recurring | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	173,785	138,887
Fair Value, Measurements, Recurring | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,432	7,242
Fair Value, Measurements, Recurring | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	1,015
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (liabilities) (level 1)
Financial assets:
Equity securities, at fair value	0	0
Investment securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	0
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2)
Financial assets:
Equity securities, at fair value	3,250	3,107
Investment securities	671,781	658,805
Loans held for sale	305,493	278,815
Mortgage servicing rights	0	0
Derivatives	29,172	14,316
Financial liabilities:
Derivatives	19,887	11,637
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	999	989
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	485,300	508,580
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	173,785	138,887
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,432	7,242
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	1,015
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3)
Financial assets:
Equity securities, at fair value	0	0
Investment securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	66,156	88,829
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3) | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3) | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3) | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3) | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	0
Fair Value, Measurements, Nonrecurring
Financial assets:
Other real estate owned	6,053	2,266
Impaired loans	5,815	8,718
Fair Value, Measurements, Nonrecurring | Commercial and industrial
Financial assets:
Impaired loans	3,479	732
Fair Value, Measurements, Nonrecurring | Consumer and other
Financial assets:
Impaired loans	395
Fair Value, Measurements, Nonrecurring | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans		832
Fair Value, Measurements, Nonrecurring | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	315	146
Fair Value, Measurements, Nonrecurring | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	260	87
Fair Value, Measurements, Nonrecurring | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans		6,921
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1)
Financial assets:
Other real estate owned	0	0
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Commercial and industrial
Financial assets:
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Consumer and other
Financial assets:
Impaired loans	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans		0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans		0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2)
Financial assets:
Other real estate owned	0	0
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Commercial and industrial
Financial assets:
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Consumer and other
Financial assets:
Impaired loans	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans		0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans		0
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3)
Financial assets:
Other real estate owned	6,053	2,266
Impaired loans	5,815	8,718
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Commercial and industrial
Financial assets:
Impaired loans	3,479	732
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Consumer and other
Financial assets:
Impaired loans	395
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans		832
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	315	146
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	$ 260	87
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans		$ 6,921