Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial assets:
Available-for-sale debt securities, at fair value	$ 655,698	$ 536,270
Equity securities, at fair value	3,107	7,722
Mortgage servicing rights		76,107	$ 33,081	$ 29,711
Derivatives	14,316	9,690
Impaired loans	13,716	9,373
Financial liabilities:
Derivatives	11,637	1,699
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	658,805	540,388
Loans held for sale	278,815	526,185
Mortgage servicing rights	0	0
Derivatives	14,316	9,690
Financial liabilities:
Derivatives	11,637	1,699
Significant unobservable inputs (level 3)
Financial assets:
Investment securities	0	3,604
Loans held for sale	0	0
Mortgage servicing rights	88,829	76,107
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring
Financial assets:
Equity securities, at fair value	3,107	7,722
Investment securities	658,805	543,992
Loans held for sale	278,815	526,185
Mortgage servicing rights	88,829	76,107
Derivatives	14,316	9,690
Financial liabilities:
Derivatives	11,637	1,699
Fair Value, Measurements, Recurring | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	989	986
Fair Value, Measurements, Recurring | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	508,580	418,781
Fair Value, Measurements, Recurring | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	138,887	109,251
Fair Value, Measurements, Recurring | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,242	7,252
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	3,107	4,118
Investment securities	658,805	540,388
Loans held for sale	278,815	526,185
Mortgage servicing rights	0	0
Derivatives	14,316	9,690
Financial liabilities:
Derivatives	11,637	1,699
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	989	986
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Mortgage-backed securities
Financial assets:
Available-for-sale debt securities, at fair value	508,580	418,781
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Municipals, tax exempt
Financial assets:
Available-for-sale debt securities, at fair value	138,887	109,251
Fair Value, Measurements, Recurring | Significant other observable inputs (level 2) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	7,242	7,252
Fair Value, Measurements, Recurring | Significant unobservable inputs (level 3)
Financial assets:
Equity securities, at fair value	0	3,604
Investment securities	0	3,604
Loans held for sale	0	0
Mortgage servicing rights	88,829	76,107
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	2,266	13,174
Impaired loans	8,718	77,027
Fair Value, Measurements, Nonrecurring | Commercial and industrial
Financial assets:
Impaired loans	732	1,971
Fair Value, Measurements, Nonrecurring | Consumer and other
Financial assets:
Impaired loans		25,320
Fair Value, Measurements, Nonrecurring | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans	832	4,211
Fair Value, Measurements, Nonrecurring | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	146	21,902
Fair Value, Measurements, Nonrecurring | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	87	10,030
Fair Value, Measurements, Nonrecurring | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans	6,921	13,593
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	2,266	13,174
Impaired loans	8,718	77,027
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Commercial and industrial
Financial assets:
Impaired loans	732	1,971
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Consumer and other
Financial assets:
Impaired loans		25,320
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Construction Loans | Commercial and industrial
Financial assets:
Impaired loans	832	4,211
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | 1-to-4 family mortgage | Residential real estate:
Financial assets:
Impaired loans	146	21,902
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Owner occupied | Commercial real estate:
Financial assets:
Impaired loans	87	10,030
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Non-owner occupied | Commercial real estate:
Financial assets:
Impaired loans	$ 6,921	$ 13,593