Fair Value of Assets and Liabilities - Additional Information about Servicing Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Servicing Assets, Changes in fair value due to:
Fair value at beginning of period			$ 21,398
Fair value at end of period	$ 29,621		29,621
Servicing Liabilities, Changes in fair value due to:
Fair value at beginning of period			2,846
Fair value at end of period	1,231		1,231
Fair Value, Measurements, Recurring
Servicing Assets, Changes in fair value due to:
Fair value at beginning of period	25,901	$ 16,126	21,398	$ 10,250
Issuances	10,563	3,009	24,460	12,984
Changes in fair value, included in servicing fees	(6,961)	(2,429)	(16,083)	(7,092)
Other net changes included in deferred revenue	118	(451)	(154)	113
Fair value at end of period	29,621	16,255	29,621	16,255
Servicing Liabilities, Changes in fair value due to:
Fair value at beginning of period	1,711	3,412	2,846	3,973
Issuances	19	712	332	2,452
Change in fair value, included in investor fees	(499)	(727)	(1,947)	(3,028)
Other net changes included in deferred revenue	0	0	0	0
Fair value at end of period	$ 1,231	$ 3,397	$ 1,231	$ 3,397