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) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans	$ 13,716	$ 9,373
Fair Value, Measurements, Nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans	8,718	77,027
Other real estate owned	2,266	13,174
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans	8,718	77,027
Other real estate owned	$ 2,266	$ 13,174
Fair Value, Measurements, Nonrecurring | Significant unobservable inputs (level 3) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans, measurement input	0.00%	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]
Impaired loans, measurement input	30.00%	30.00%
Other real estate owned, measurement input	0.15	0.15