Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,829	$ 3,332	$ 3,529	$ 3,273
Provision for loan repurchases or indemnifications	855	89	1,227	148
Recoveries on previous losses	(83)	(14)	(155)	(14)
Balance at end of period	$ 4,601	$ 3,407	$ 4,601	$ 3,407