Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 4,601	$ 3,407	$ 3,529	$ 3,273
Provision for loan repurchases or indemnifications	901	107	2,128	255
Recoveries on previous losses	(44)	(79)	(199)	(93)
Balance at end of period	$ 5,458	$ 3,435	$ 5,458	$ 3,435