Commitments and Contingencies - Summary of Allowance for Loan Repurchases or Indemnifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Balance at beginning of period	$ 3,514	$ 2,842	$ 3,386	$ 2,659
Provision for loan repurchases or indemnifications	206	201	392	384
Losses on loans repurchased or indemnified	(74)	(6)	(132)	(6)
Balance at end of period	$ 3,646	$ 3,037	$ 3,646	$ 3,037