Commitments and contingencies - Summary of Financial Instruments with Off-Balance Sheet Credit Risk (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	$ 1,174,172	$ 1,105,742
Commitments to extend credit, excluding interest rate lock commitments
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	1,154,092	1,086,173
Letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	$ 20,080	$ 19,569