Fair Value of Assets and Liabilities - Additional Information about Loans, Notes and Certificates Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Loans
Outstanding Principal Balance, Beginning	$ 3,694,823	$ 2,351,515	$ 2,836,729	$ 1,849,042
Valuation Adjustment, Beginning	(57,440)	(25,313)	(38,224)	(20,000)
Fair Value, Beginning	3,637,383	2,326,202	2,798,505	1,829,042
Outstanding Principal Balance, Purchases of loans	1,946,455	994,497	5,075,044	2,628,758
Valuation Adjustment, Purchases of loans	0	0	0	0
Fair Value, Purchases of loans	1,946,455	994,497	5,075,044	2,628,758
Outstanding Principal Balance, Issuances of notes and certificates	0	0	0	0
Valuation Adjustment, Issuances of notes and certificates	0	0	0	0
Fair Value, Issuances of notes and certificates	0	0	0	0
Outstanding Principal Balance, Whole loan sales	(954,770)	(462,523)	(2,338,346)	(1,094,482)
Valuation Adjustment, Whole loan sales	0	0	0	0
Fair Value, Whole loan sales	(954,770)	(462,523)	(2,338,346)	(1,094,482)
Outstanding Principal Balance, Principal payments	(481,701)	(288,102)	(1,280,005)	(739,506)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments	(481,701)	(288,102)	(1,280,005)	(739,506)
Outstanding Principal Balance, Charge-offs	(55,365)	(28,910)	(143,980)	(77,335)
Asset Valuation Adjustment, Charge-offs	55,365	28,910	143,980	77,335
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(5,919)	(2,594)	(13,729)	(5,178)
Fair Value, Recoveries	(5,919)	(2,594)	(13,729)	(5,178)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(72,474)	(33,809)	(172,495)	(84,963)
Fair Value, Change in fair value recorded in earnings	(72,474)	(33,809)	(172,495)	(84,963)
Outstanding Principal Balance, Ending	4,149,442	2,566,477	4,149,442	2,566,477
Valuation Adjustment, Ending	(80,468)	(32,806)	(80,468)	(32,806)
Fair Value, Ending	4,068,974	2,533,671	4,068,974	2,533,671
Notes and Certificates
Outstanding Principal Balance, Beginning	3,717,556	2,361,902	2,851,837	1,859,982
Valuation Adjustment, Beginning	(57,432)	(25,307)	(38,219)	(19,992)
Fair Value, Beginning	3,660,124	2,336,595	2,813,618	1,839,990
Outstanding Principal Balance, Purchases of loans	0	0	0	0
Valuation Adjustment, Purchases of loans	0	0	0	0
Fair Value, Purchases of loans	0	0	0	0
Outstanding Principal Balance, Issuances of notes and certificates	991,926	532,034	2,736,667	1,534,011
Valuation Adjustment, Issuances of notes and certificates	0	0	0	0
Fair Value, Issuances of notes and certificates	991,926	532,034	2,736,667	1,534,011
Outstanding Principal Balance, Whole loan sales	0	0	0	0
Valuation Adjustment, Whole loan sales	0	0	0	0
Fair Value, Whole loan sales	0	0	0	0
Outstanding Principal Balance, Principal payments	(478,189)	(280,643)	(1,268,622)	(732,343)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments	(478,189)	(280,643)	(1,268,622)	(732,343)
Outstanding Principal Balance, Recovers	0	0	0	0
Valuation Adjustment, Recoveries	(5,867)	(2,589)	(13,653)	(5,153)
Fair Value, Recoveries	(5,867)	(2,589)	(13,653)	(5,153)
Outstanding Principal Balance, Charge-offs	(55,346)	(28,852)	(143,935)	(77,209)
Valuation Adjustment, Charge-offs	55,346	28,852	143,935	77,209
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(72,513)	(33,757)	(172,529)	(84,865)
Fair Value, Change in fair value recorded in earnings	(72,513)	(33,757)	(172,529)	(84,865)
Outstanding Principal Balance, Ending	4,175,947	2,584,441	4,175,947	2,584,441
Valuation Adjustment, Ending	(80,466)	(32,801)	(80,466)	(32,801)
Fair Value, Ending	$ 4,095,481	$ 2,551,640	$ 4,095,481	$ 2,551,640