Commitments and contingencies - Summary of Financial Instruments with Off-Balance Sheet Credit Risk (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	$ 1,210,536	$ 1,051,414
Commitments to extend credit, excluding interest rate lock commitments
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	1,188,160	1,032,390
Letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	$ 22,376	$ 19,024