Fair value of financial instruments - Information about Significant Unobservable Inputs (Level 3) Used in Valuation of Assets Measured at Fair Value on Nonrecurring Basis (Details) $ in Thousands	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans		$ 23,810
Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans	$ 7,270	10,691
Other real estate owned	1,058	9,774
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Collateral dependent loans	7,270
Impaired loans	7,270	10,691
Other real estate owned	$ 1,058	$ 9,774
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Collateral dependent loans, measurement input	0.00
Impaired loans, measurement input		0.00%
Other real estate owned, measurement input	0.00	0.00
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Collateral dependent loans, measurement input	0.30
Impaired loans, measurement input		30.00%
Other real estate owned, measurement input	0.15	0.15