Commitments and contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,386	$ 2,659	$ 2,156
Provision for loan repurchases or indemnifications	174	810	512
Recoveries on previous losses	3	0	9
Losses on loans repurchased or indemnified	(290)	(83)	(18)
Balance at end of period	$ 3,273	$ 3,386	$ 2,659