Commitments and Contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,386	$ 2,659
Provision for loan repurchases or indemnifications	186	183
Recoveries on previous losses	(58)
Balance at end of period	$ 3,514	$ 2,842