Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial assets:
Available-for-sale debt securities, at fair value	$ 764,217	$ 688,381
Equity securities, at fair value	3,358	3,295
Collateral dependent loans		23,810
Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	235,677	189,235
Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	24,860	7,448
Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	985	1,022
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	767,575	691,676
Loans held for sale	325,304	262,518
Mortgage servicing rights	0	0
Derivatives	66,281	21,981
Financial liabilities:
Derivatives	66,696	17,933
Significant unobservable inputs (level 3)
Financial assets:
Investment securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	62,581	75,521
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring
Financial assets:
Equity securities, at fair value	3,358	3,295
Investment securities	767,575	691,676
Loans held for sale	325,304	262,518
Mortgage servicing rights	62,581	75,521
Derivatives	66,281	21,981
Financial liabilities:
Derivatives	66,696	17,933
Fair Value, Measurements, Recurring | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	3,037
Fair Value, Measurements, Recurring | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	499,658	490,676
Fair Value, Measurements, Recurring | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	235,677	189,235
Fair Value, Measurements, Recurring | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	24,860	7,448
Fair Value, Measurements, Recurring | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	985	1,022
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
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	0
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2)
Financial assets:
Equity securities, at fair value	3,358	3,295
Investment securities	767,575	691,676
Loans held for sale	325,304	262,518
Mortgage servicing rights	0	0
Derivatives	66,281	21,981
Financial liabilities:
Derivatives	66,696	17,933
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	3,037
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	499,658	490,676
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	235,677	189,235
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	24,860	7,448
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	985	1,022
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	62,581	75,521
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
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	0
Fair Value, Measurements, Nonrecurring
Financial assets:
Other real estate owned	1,058	9,774
Collateral dependent loans	7,270	10,691
Fair Value, Measurements, Nonrecurring | Commercial and industrial
Financial assets:
Collateral dependent loans	1,566	6,481
Fair Value, Measurements, Nonrecurring | Residential line of credit
Financial assets:
Collateral dependent loans	311	321
Fair Value, Measurements, Nonrecurring | Consumer and other
Financial assets:
Collateral dependent loans	0
Fair Value, Measurements, Nonrecurring | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Collateral dependent loans	0	378
Fair Value, Measurements, Nonrecurring | Owner occupied | Commercial real estate:
Financial assets:
Collateral dependent loans	0	951
Fair Value, Measurements, Nonrecurring | Non-owner occupied | Commercial real estate:
Financial assets:
Collateral dependent loans	5,704	2,560
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1)
Financial assets:
Other real estate owned	0	0
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Commercial and industrial
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Residential line of credit
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Quoted prices in active markets for identical assets (liabilities) (level 1) | Consumer and other
Financial assets:
Collateral dependent 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:
Collateral dependent 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:
Collateral dependent 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:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2)
Financial assets:
Other real estate owned	0	0
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Commercial and industrial
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Residential line of credit
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Consumer and other
Financial assets:
Collateral dependent loans	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Owner occupied | Commercial real estate:
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant other observable inputs (level 2) | Non-owner occupied | Commercial real estate:
Financial assets:
Collateral dependent loans	0	0
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3)
Financial assets:
Other real estate owned	1,058	9,774
Collateral dependent loans	7,270	10,691
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Commercial and industrial
Financial assets:
Collateral dependent loans	1,566	6,481
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Residential line of credit
Financial assets:
Collateral dependent loans	0	321
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Consumer and other
Financial assets:
Collateral dependent loans	0
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Collateral dependent loans	0	378
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Owner occupied | Commercial real estate:
Financial assets:
Collateral dependent loans	0	951
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Non-owner occupied | Commercial real estate:
Financial assets:
Collateral dependent loans	$ 5,704	$ 2,560