Commitments and Contingencies - Additional Information (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]
Total principal amount of loans repurchased or indemnified	$ 2,322	$ 1,137	$ 4,984	$ 2,907