Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Available-for-sale debt securities, at fair value	$ 1,172,400	$ 688,381
Equity securities, at fair value	4,591	3,295
Derivatives	68,938	21,981
Financial liabilities:
Derivatives	48,242	17,933
Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	773,336	477,312
Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	21,588	13,364
U.S. Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	16,628	7,448
Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	2,516	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	1,176,991	691,676
Loans held for sale	683,770	262,518
Mortgage servicing rights	0	0
Derivatives	68,938	21,981
Financial liabilities:
Derivatives	48,242	17,933
Significant unobservable inputs (level 3)
Financial assets:
Investment securities	0	0
Loans held for sale	215,403	0
Mortgage servicing rights	79,997	75,521
Derivatives	0	0
Financial liabilities:
Derivatives	0	0
Fair Value, Measurements, Recurring
Financial assets:
Equity securities, at fair value	4,591	3,295
Investment securities	1,176,991	691,676
Loans held for sale	899,173	262,518
Mortgage servicing rights	79,997	75,521
Derivatives	68,938	21,981
Financial liabilities:
Derivatives	48,242	17,933
Fair Value, Measurements, Recurring \| U.S. government agency securities
Financial assets:
Available-for-sale debt securities, at fair value	2,003
Fair Value, Measurements, Recurring \| Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	773,336	477,312
Fair Value, Measurements, Recurring \| Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	21,588	13,364
Fair Value, Measurements, Recurring \| Municipals, tax-exempt
Financial assets:
Available-for-sale debt securities, at fair value	356,329	189,235
Fair Value, Measurements, Recurring \| U.S. Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	16,628	7,448
Fair Value, Measurements, Recurring \| Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	2,516	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 - residential
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 - commercial
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) \| U.S. 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	4,591	3,295
Investment securities	1,176,991	691,676
Loans held for sale	683,770	262,518
Mortgage servicing rights	0	0
Derivatives	68,938	21,981
Financial liabilities:
Derivatives	48,242	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	2,003
Fair Value, Measurements, Recurring \| Significant other observable inputs (level 2) \| Mortgage-backed securities - residential
Financial assets:
Available-for-sale debt securities, at fair value	773,336	477,312
Fair Value, Measurements, Recurring \| Significant other observable inputs (level 2) \| Mortgage-backed securities - commercial
Financial assets:
Available-for-sale debt securities, at fair value	21,588	13,364
Fair Value, Measurements, Recurring \| Significant other observable inputs (level 2) \| Municipals, tax-exempt
Financial assets:
Available-for-sale debt securities, at fair value	356,329	189,235
Fair Value, Measurements, Recurring \| Significant other observable inputs (level 2) \| U.S. Treasury securities
Financial assets:
Available-for-sale debt securities, at fair value	16,628	7,448
Fair Value, Measurements, Recurring \| Significant other observable inputs (level 2) \| Corporate securities
Financial assets:
Available-for-sale debt securities, at fair value	2,516	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	215,403	0
Mortgage servicing rights	79,997	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 - residential
Financial assets:
Available-for-sale debt securities, at fair value	0	0
Fair Value, Measurements, Recurring \| Significant unobservable inputs (level 3) \| Mortgage-backed securities - commercial
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) \| U.S. 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

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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