Fair Value of Assets and Liabilities - Additional Information about Servicing Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Servicing Assets
Servicing Assets, Fair value, Beginning Balance	$ 21,398
Servicing Assets, Fair value, Ending Balance	33,676	$ 21,398
Servicing Liabilities
Servicing Liabilities, Fair value, Beginning Balance	2,846
Servicing Liabilities, Fair value, Ending Balance	833	2,846
Fair Value, Measurements, Recurring
Servicing Assets
Servicing Assets, Fair value, Beginning Balance	21,398	10,250
Issuances	34,950	16,546
Changes in fair value, included in investor fees	(23,172)	(5,403)
Other net changes included in deferred revenue	500	5
Servicing Assets, Fair value, Ending Balance	33,676	21,398
Servicing Liabilities
Servicing Liabilities, Fair value, Beginning Balance	2,846	3,973
Issuances	333	3,371
Changes in fair value, included in investor fees	(2,346)	(4,498)
Other net changes included in deferred revenue	0	0
Servicing Liabilities, Fair value, Ending Balance	$ 833	$ 2,846