Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,273	$ 3,386
Provision for loan repurchases or indemnifications	59	186
Recoveries on previous losses	0	(58)
Balance at end of period	$ 3,332	$ 3,514