Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,273	$ 3,386	$ 2,659
Provision for loan repurchases or indemnifications	362	174	810
Recoveries on previous losses	(106)	3	0
Losses on loans repurchased or indemnified	0	(290)	(83)
Balance at end of period	$ 3,529	$ 3,273	$ 3,386