Commitments and Contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,646	$ 3,037	$ 3,386	$ 2,659
Provision for loan repurchases or indemnifications	206	410	598	794
Losses on loans repurchased or indemnified	(115)	(27)	(247)	(33)
Balance at end of period	$ 3,737	$ 3,420	$ 3,737	$ 3,420