Fair Value of Assets and Liabilities - Additional Information about Loans, Notes, Certificates and Secured Borrowings Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans Invested in by Company
Loans Held For Investment
Outstanding Principal Balance, Beginning	$ 27,860	$ 3,462
Valuation adjustment, Beginning	(1,949)	(4)
Fair Value, Beginning	25,911	3,458
Outstanding Principal Balance, Purchases of loans	1,648,297	183,838
Valuation Adjustment, Purchases of loans	(899)	(656)
Fair Value, Purchases of loans	1,647,398	183,182
Transfers (to) from loans held for investment (from) to loans held for sale	0	0
Valuation Adjustment, Transfers from loans held for investment to loans held for sale	0	0
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale	0	0
Outstanding Principal Balance, Sales	(990,267)	(144,655)
Valuation Adjustment, Sales	5,871	0
Fair Value, Sales	(984,396)	(144,655)
Outstanding Principal Balance, Principal payments	(65,681)	(12,632)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments	(65,681)	(12,632)
Outstanding Principal Balance, Charge-offs	(6,557)	(2,153)
Valuation Adjustment, Charge-offs	6,557	2,153
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(343)	(493)
Fair Value, Recoveries	(343)	(493)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(25,834)	(2,949)
Fair Value, Change in fair value recorded in earnings	(25,834)	(2,949)
Outstanding Principal Balance, Ending	613,652	27,860
Valuation adjustment, Ending	(16,597)	(1,949)
Fair Value, Ending	597,055	25,911
Notes, Certificates and Secured Borrowings
Notes, Certificates and Secured Borrowings
Outstanding Principal Balance, Beginning	4,572,912	4,697,169
Valuation adjustment, Beginning	(252,017)	(125,586)
Fair value at beginning of period	4,320,895	4,571,583
Outstanding Principal Balance, Purchases of loans	0	0
Valuation Adjustment, Purchases of loans	0	0
Fair Value, Purchases of loans	0	0
Outstanding Principal Balance, Transfers from loans to loans held for sale	0	0
Valuation Adjustment, Transfers from loans to loans held for sale	0	0
Fair Value, Transfers from loans to loans held for sale	0	0
Outstanding Principal Balance, Issuances of notes and certificates	2,019,316	2,681,109
Valuation Adjustment, Issuances of notes and certificates	(17,937)	0
Fair Value, Issuances of notes and certificates	2,001,379	2,681,109
Outstanding Principal Balance, Principal payments	0	0
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments	0	0
Outstanding Principal Balance, Whole loan sales	(2,941,692)	(2,385,234)
Valuation Adjustment, Whole loan sales	31,606	0
Fair Value, Whole loan sales	(2,910,086)	(2,385,234)
Outstanding Principal Balance, Charge-offs	(489,456)	(420,132)
Valuation Adjustment, Charge-offs	489,456	420,132
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(47,914)	(36,785)
Fair Value, Recoveries	(47,914)	(36,785)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(409,506)	(509,778)
Fair Value, Change in fair value recorded in earnings	(409,506)	(509,778)
Outstanding Principal Balance, Ending	3,161,080	4,572,912
Valuation adjustment, Ending	(206,312)	(252,017)
Fair value at end of period	2,954,768	4,320,895
Loans Held for Investment
Loans Held For Investment
Outstanding Principal Balance, Beginning	4,547,138	4,678,209
Valuation adjustment, Beginning	(252,017)	(125,586)
Fair Value, Beginning	4,295,121	4,552,623
Outstanding Principal Balance, Purchases of loans	1,720,343	2,653,589
Valuation Adjustment, Purchases of loans	5	0
Fair Value, Purchases of loans	1,720,348	2,653,589
Outstanding Principal Balance, Issuances	0	0
Valuation Adjustment, Issuances	0	0
Fair Value, Issuances	0	0
Transfers (to) from loans held for investment (from) to loans held for sale	(253,124)	20,573
Valuation Adjustment, Transfers from loans held for investment to loans held for sale	(4,112)	0
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale	(257,236)	20,573
Outstanding Principal Balance, Sales	0	0
Valuation Adjustment, Sales	0	0
Fair Value, Sales	0	0
Outstanding Principal Balance, Principal payments	(2,383,510)	(2,385,102)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments	(2,383,510)	(2,385,102)
Outstanding Principal Balance, Charge-offs	(489,456)	(420,131)
Valuation Adjustment, Charge-offs	489,456	420,131
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(47,913)	(36,784)
Fair Value, Recoveries	(47,913)	(36,784)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(394,485)	(509,778)
Fair Value, Change in fair value recorded in earnings	(394,485)	(509,778)
Outstanding Principal Balance, Ending	3,141,391	4,547,138
Valuation adjustment, Ending	(209,066)	(252,017)
Fair Value, Ending	2,932,325	4,295,121
Loans Held for Investment | Loans Invested in by Company
Loans Held For Investment
Outstanding Principal Balance, Beginning	18,515	3,462
Valuation adjustment, Beginning	(1,652)	(4)
Fair Value, Beginning	16,863	3,458
Outstanding Principal Balance, Purchases of loans	19,069	79,736
Valuation Adjustment, Purchases of loans	(707)	(656)
Fair Value, Purchases of loans	18,362	79,080
Transfers (to) from loans held for investment (from) to loans held for sale	354,410	(55,984)
Valuation Adjustment, Transfers from loans held for investment to loans held for sale	4,112	0
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale	358,522	(55,984)
Outstanding Principal Balance, Sales	0	0
Valuation Adjustment, Sales	0	0
Fair Value, Sales	0	0
Outstanding Principal Balance, Principal payments	(16,433)	(6,705)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments	(16,433)	(6,705)
Outstanding Principal Balance, Charge-offs	(4,182)	(1,994)
Valuation Adjustment, Charge-offs	4,182	1,994
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(343)	(493)
Fair Value, Recoveries	(343)	(493)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(15,741)	(2,493)
Fair Value, Change in fair value recorded in earnings	(15,741)	(2,493)
Outstanding Principal Balance, Ending	371,379	18,515
Valuation adjustment, Ending	(10,149)	(1,652)
Fair Value, Ending	361,230	16,863
Loans Held For Sale
Loans Held For Investment
Outstanding Principal Balance, Beginning	0	0
Valuation adjustment, Beginning	0	0
Fair Value, Beginning	0	0
Outstanding Principal Balance, Purchases of loans	5,232,503	4,638,436
Valuation Adjustment, Purchases of loans	6,420	0
Fair Value, Purchases of loans	5,238,923	4,638,436
Outstanding Principal Balance, Issuances	0	0
Valuation Adjustment, Issuances	0	0
Fair Value, Issuances	0	0
Transfers (to) from loans held for investment (from) to loans held for sale	253,124	(20,573)
Valuation Adjustment, Transfers from loans held for investment to loans held for sale	4,112	0
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale	257,236	(20,573)
Outstanding Principal Balance, Sales	(5,483,146)	(4,617,863)
Valuation Adjustment, Sales	8,067	0
Fair Value, Sales	(5,475,079)	(4,617,863)
Outstanding Principal Balance, Principal payments	(2,481)	0
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments	(2,481)	0
Outstanding Principal Balance, Charge-offs	0	0
Valuation Adjustment, Charge-offs	0	0
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	0	0
Fair Value, Recoveries	0	0
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(18,599)	0
Fair Value, Change in fair value recorded in earnings	(18,599)	0
Outstanding Principal Balance, Ending	0	0
Valuation adjustment, Ending	0	0
Fair Value, Ending	0	0
Loans Held For Sale | Loans Invested in by Company
Loans Held For Investment
Outstanding Principal Balance, Beginning	9,345	0
Valuation adjustment, Beginning	(297)	0
Fair Value, Beginning	9,048	0
Outstanding Principal Balance, Purchases of loans	1,629,228	104,102
Valuation Adjustment, Purchases of loans	(192)	0
Fair Value, Purchases of loans	1,629,036	104,102
Transfers (to) from loans held for investment (from) to loans held for sale	(354,410)	55,984
Valuation Adjustment, Transfers from loans held for investment to loans held for sale	(4,112)	0
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale	(358,522)	55,984
Outstanding Principal Balance, Sales	(990,267)	(144,655)
Valuation Adjustment, Sales	5,871	0
Fair Value, Sales	(984,396)	(144,655)
Outstanding Principal Balance, Principal payments	(49,248)	(5,927)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments	(49,248)	(5,927)
Outstanding Principal Balance, Charge-offs	(2,375)	(159)
Valuation Adjustment, Charge-offs	2,375	159
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	0	0
Fair Value, Recoveries	0	0
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(10,093)	(456)
Fair Value, Change in fair value recorded in earnings	(10,093)	(456)
Outstanding Principal Balance, Ending	242,273	9,345
Valuation adjustment, Ending	(6,448)	(297)
Fair Value, Ending	$ 235,825	$ 9,048