Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,332	$ 3,514	$ 3,273	$ 3,386
Provision for loan repurchases or indemnifications	89	206	148	392
Recoveries on previous losses	(14)	(74)	(14)	(132)
Balance at end of period	$ 3,407	$ 3,646	$ 3,407	$ 3,646