Fair value of financial instruments - Balances and Levels of Assets Measured at Fair Value on Nonrecurring Basis (Details) - Non-recurring Basis - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financial assets:
Other real estate owned	$ 3,870	$ 6,662
Collateral dependent loans	47,430	18,560
Commercial and industrial
Financial assets:
Collateral dependent loans	13,219	1,728
Construction
Financial assets:
Collateral dependent loans	5,513	3,877
Residential real estate | 1-to-4 family mortgage
Financial assets:
Collateral dependent loans	1,725	226
Residential real estate | Residential line of credit
Financial assets:
Collateral dependent loans	1,256	1,174
Residential real estate | Multifamily
Financial assets:
Collateral dependent loans	485
Commercial real estate | Owner occupied
Financial assets:
Collateral dependent loans	13,451	3,391
Commercial real estate | Non-owner occupied
Financial assets:
Collateral dependent loans	11,781	8,164
Quoted prices in active markets for identical assets (liabilities) (level 1)
Financial assets:
Other real estate owned	0	0
Collateral dependent loans	0	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Commercial and industrial
Financial assets:
Collateral dependent loans	0	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Construction
Financial assets:
Collateral dependent loans	0	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Residential real estate | 1-to-4 family mortgage
Financial assets:
Collateral dependent loans	0	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Residential real estate | Residential line of credit
Financial assets:
Collateral dependent loans	0	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Residential real estate | Multifamily
Financial assets:
Collateral dependent loans	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Commercial real estate | Owner occupied
Financial assets:
Collateral dependent loans	0	0
Quoted prices in active markets for identical assets (liabilities) (level 1) | Commercial real estate | Non-owner occupied
Financial assets:
Collateral dependent loans	0	0
Significant other observable inputs (level 2)
Financial assets:
Other real estate owned	0	0
Collateral dependent loans	0	0
Significant other observable inputs (level 2) | Commercial and industrial
Financial assets:
Collateral dependent loans	0	0
Significant other observable inputs (level 2) | Construction
Financial assets:
Collateral dependent loans	0	0
Significant other observable inputs (level 2) | Residential real estate | 1-to-4 family mortgage
Financial assets:
Collateral dependent loans	0	0
Significant other observable inputs (level 2) | Residential real estate | Residential line of credit
Financial assets:
Collateral dependent loans	0	0
Significant other observable inputs (level 2) | Residential real estate | Multifamily
Financial assets:
Collateral dependent loans	0
Significant other observable inputs (level 2) | Commercial real estate | Owner occupied
Financial assets:
Collateral dependent loans	0	0
Significant other observable inputs (level 2) | Commercial real estate | Non-owner occupied
Financial assets:
Collateral dependent loans	0	0
Significant unobservable inputs (level 3)
Financial assets:
Other real estate owned	3,870	6,662
Collateral dependent loans	47,430	18,560
Significant unobservable inputs (level 3) | Commercial and industrial
Financial assets:
Collateral dependent loans	13,219	1,728
Significant unobservable inputs (level 3) | Construction
Financial assets:
Collateral dependent loans	5,513	3,877
Significant unobservable inputs (level 3) | Residential real estate | 1-to-4 family mortgage
Financial assets:
Collateral dependent loans	1,725	226
Significant unobservable inputs (level 3) | Residential real estate | Residential line of credit
Financial assets:
Collateral dependent loans	1,256	1,174
Significant unobservable inputs (level 3) | Residential real estate | Multifamily
Financial assets:
Collateral dependent loans	485
Significant unobservable inputs (level 3) | Commercial real estate | Owner occupied
Financial assets:
Collateral dependent loans	13,451	3,391
Significant unobservable inputs (level 3) | Commercial real estate | Non-owner occupied
Financial assets:
Collateral dependent loans	$ 11,781	$ 8,164