Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Financial assets:
Available-for-sale debt securities, at fair value	$ 1,678,525	$ 1,172,400
Equity securities, at fair value	3,367	4,591
Loans held for sale	752,223	899,173
Derivatives	27,384	68,938
Financial liabilities:
Derivatives	21,000	48,242
Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	1,269,372	773,336
Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	15,250	21,588
Municipal securities
Financial assets:
Available-for-sale debt securities, at fair value	338,610	356,329
Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	14,908	16,628
Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	6,515	2,516
Recurring Basis
Financial assets:
Equity securities, at fair value	3,367	4,591
Total securities	1,681,892	1,176,991
Loans held for sale	752,223	899,173
Mortgage servicing rights	115,512	79,997
Derivatives	27,384	68,938
Financial liabilities:
Derivatives	21,000	48,242
Recurring Basis | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	33,870	2,003
Recurring Basis | Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	1,269,372	773,336
Recurring Basis | Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	15,250	21,588
Recurring Basis | Municipal securities
Financial assets:
Available-for-sale debt securities, at fair value	338,610	356,329
Recurring Basis | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	14,908	16,628
Recurring Basis | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	6,515	2,516
Recurring Basis | Quoted prices in active markets for identical assets (liabilities) (level 1)
Financial assets:
Equity securities, at fair value	0	0
Total securities	0	0
Loans held for sale	0	0
Mortgage servicing rights	0	0
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Recurring Basis | 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
Recurring Basis | Quoted prices in active markets for identical assets (liabilities) (level 1) | Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Quoted prices in active markets for identical assets (liabilities) (level 1) | Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Quoted prices in active markets for identical assets (liabilities) (level 1) | Municipal securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | 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
Recurring Basis | 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
Recurring Basis | Significant other observable inputs (level 2)
Financial assets:
Equity securities, at fair value	3,367	4,591
Total securities	1,681,892	1,176,991
Loans held for sale	672,924	683,770
Mortgage servicing rights	0	0
Derivatives	27,384	68,938
Financial liabilities:
Derivatives	21,000	48,242
Recurring Basis | Significant other observable inputs (level 2) | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	33,870	2,003
Recurring Basis | Significant other observable inputs (level 2) | Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	1,269,372	773,336
Recurring Basis | Significant other observable inputs (level 2) | Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	15,250	21,588
Recurring Basis | Significant other observable inputs (level 2) | Municipal securities
Financial assets:
Available-for-sale debt securities, at fair value	338,610	356,329
Recurring Basis | Significant other observable inputs (level 2) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	14,908	16,628
Recurring Basis | Significant other observable inputs (level 2) | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	6,515	2,516
Recurring Basis | Significant unobservable inputs (level 3)
Financial assets:
Equity securities, at fair value	0	0
Total securities	0	0
Loans held for sale	79,299	215,403
Mortgage servicing rights	115,512	79,997
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Recurring Basis | Significant unobservable inputs (level 3) | U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Significant unobservable inputs (level 3) | Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Significant unobservable inputs (level 3) | Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Significant unobservable inputs (level 3) | Municipal securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Significant unobservable inputs (level 3) | Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Recurring Basis | Significant unobservable inputs (level 3) | Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	$ 0	$ 0