Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,407	$ 3,646	$ 3,273	$ 3,386
Provision for loan repurchases or indemnifications	107	206	255	598
Recoveries on previous losses	(79)	(115)	(93)	(247)
Balance at end of period	$ 3,435	$ 3,737	$ 3,435	$ 3,737