Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial assets:
Available-for-sale debt securities, at fair value	$ 675,215	$ 655,698
Equity securities, at fair value	3,242	3,107
Impaired loans	15,678	13,716
Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	149,305	138,887
Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,409	7,242
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	678,457	658,805
Loans held for sale	294,699	278,815
Mortgage servicing rights	0	0
Derivatives	24,407	14,316
Financial liabilities:
Derivatives	19,421	11,637
Significant unobservable inputs (level 3)
Financial assets:
Investment securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	66,380	88,829
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring
Financial assets:
Equity securities, at fair value	3,242	3,107
Investment securities	678,457	658,805
Loans held for sale	294,699	278,815
Mortgage servicing rights	66,380	88,829
Derivatives	24,407	14,316
Financial liabilities:
Derivatives	19,421	11,637
Fair Value, Measurements, Recurring | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	996	989
Fair Value, Measurements, Recurring | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	517,505	508,580
Fair Value, Measurements, Recurring | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	149,305	138,887
Fair Value, Measurements, Recurring | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,409	7,242
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 | Significant other observable inputs (level 2)
Financial assets:
Equity securities, at fair value	3,242	3,107
Investment securities	678,457	658,805
Loans held for sale	294,699	278,815
Mortgage servicing rights	0	0
Derivatives	24,407	14,316
Financial liabilities:
Derivatives	19,421	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	996	989
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	517,505	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	149,305	138,887
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,409	7,242
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	66,380	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, Nonrecurring
Financial assets:
Other real estate owned	4,613	2,266
Impaired loans	4,424	8,718
Fair Value, Measurements, Nonrecurring | Commercial and industrial
Financial assets:
Impaired loans	3,528	732
Fair Value, Measurements, Nonrecurring | Consumer and other
Financial assets:
Impaired loans	499
Fair Value, Measurements, Nonrecurring | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans	0	832
Fair Value, Measurements, Nonrecurring | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	317	146
Fair Value, Measurements, Nonrecurring | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	80	87
Fair Value, Measurements, Nonrecurring | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans	0	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	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	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	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	0
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3)
Financial assets:
Other real estate owned	4,613	2,266
Impaired loans	4,424	8,718
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Commercial and industrial
Financial assets:
Impaired loans	3,528	732
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Consumer and other
Financial assets:
Impaired loans	499
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans	0	832
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	317	146
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	80	87
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans	$ 0	$ 6,921