Commitments and contingencies - Summary of Financial Instruments with Off-Balance Sheet Credit Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	$ 1,051,414	$ 1,000,158
Commitments to extend credit, excluding interest rate lock commitments
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	1,032,390	977,276
Letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Balance at end of period	$ 19,024	$ 22,882