Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,529	$ 3,273
Provision for loan repurchases or indemnifications	372	59
Recoveries on previous losses	(72)	0
Balance at end of period	$ 3,829	$ 3,332