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, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans Invested in by Company
Loans
Outstanding principal balance, beginning	$ 794,436	$ 873,233
Valuation adjustment, beginning	(28,388)	(30,629)
Fair value, beginning		842,604
Outstanding principal balance, purchases of loans	1,570,279	5,346,139
Valuation adjustment, purchases of loans	(1,302)	(2,302)
Fair value, purchases of loans	1,568,977	5,343,837
Transfers (to) from loans held for investment and/or loans held for sale	503	706
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	503	706
Outstanding principal balance, sales	(1,907,446)	(5,122,450)
Valuation adjustment, sales	87,723	119,369
Fair value, sales	(1,819,723)	(5,003,081)
Outstanding principal balance, principal payments	(237,123)	(273,580)
Valuation adjustment, principal payments	0	0
Fair value, principal payments	(237,123)	(273,580)
Outstanding principal balance, charge-offs	(31,661)	(29,612)
Valuation adjustment, charge-offs	26,533	26,142
Fair value, charge-offs	(5,128)	(3,470)
Outstanding principal balance, change in fair value recorded in earnings	0	0
Valuation adjustment, change in fair value recorded in earnings	(101,698)	(140,968)	$ (102,000)
Fair value, change in fair value recorded in earnings	(101,698)	(140,968)
Outstanding principal balance, ending	188,988	794,436	873,233
Valuation adjustment, ending	(17,132)	(28,388)	(30,629)
Fair value, ending	171,856		842,604
Notes, Certificates and Secured Borrowings
Notes, Certificates and Secured Borrowings
Outstanding principal balance, beginning	1,148,888	2,033,258
Valuation adjustment, beginning	(67,422)	(127,383)
Fair value at beginning of period	1,081,466	1,905,875
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	314,995	632,962
Valuation adjustment, issuances of notes and certificates	0	0
Fair value, issuances of notes and certificates	314,995	632,962
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	(705,639)	(1,326,526)
Valuation adjustment, whole loan sales	0	14
Fair value, whole loan sales	(705,639)	(1,326,512)
Outstanding principal balance, charge-offs	(78,341)	(190,806)
Valuation adjustment, charge-offs	37,161	135,785
Fair value, charge-offs	(41,180)	(55,021)
Outstanding principal balance, change in fair value recorded in earnings	0	0
Valuation adjustment, change in fair value recorded in earnings	(12,868)	(75,838)
Fair value, change in fair value recorded in earnings	(12,868)	(75,838)
Outstanding principal balance, ending	679,903	1,148,888	2,033,258
Valuation adjustment, ending	(43,129)	(67,422)	(127,383)
Fair value at end of period	636,774	1,081,466	1,905,875
Loans Held for Investment
Loans
Outstanding principal balance, beginning	1,148,888	2,013,438
Valuation adjustment, beginning	(69,573)	(130,187)
Fair value, beginning	1,079,315	1,883,251
Outstanding principal balance, purchases of loans	314,995	632,962
Valuation adjustment, purchases of loans	0	(21)
Fair value, purchases of loans	314,995	632,941
Transfers (to) from loans held for investment and/or loans held for sale	(17,916)	(123,036)
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	(17,916)	(123,036)
Outstanding principal balance, issuances	0	0
Valuation adjustment, issuances	0	0
Fair value, issuances	0	0
Outstanding principal balance, sales	0	0
Valuation adjustment, sales	0	0
Fair value, sales	0	0
Outstanding principal balance, principal payments	(687,723)	(1,183,670)
Valuation adjustment, principal payments	0	0
Fair value, principal payments	(687,723)	(1,183,670)
Outstanding principal balance, charge-offs	(78,341)	(190,806)
Valuation adjustment, charge-offs	39,641	138,857
Fair value, charge-offs	(38,700)	(51,949)
Outstanding principal balance, change in fair value recorded in earnings	0	0
Valuation adjustment, change in fair value recorded in earnings	(13,285)	(78,222)
Fair value, change in fair value recorded in earnings	(13,285)	(78,222)
Outstanding principal balance, ending	679,903	1,148,888	2,013,438
Valuation adjustment, ending	(43,217)	(69,573)	(130,187)
Fair value, ending	636,686	1,079,315	1,883,251
Loans Held for Investment | Loans Invested in by Company
Loans
Outstanding principal balance, beginning	47,042	3,518
Valuation adjustment, beginning	(3,349)	(935)
Fair value, beginning	43,693	2,583
Outstanding principal balance, purchases of loans	1,435	2,993
Valuation adjustment, purchases of loans	(1,296)	(2,303)
Fair value, purchases of loans	139	690
Transfers (to) from loans held for investment and/or loans held for sale	41,934	49,996
Valuation adjustment, transfers from loans held for investment to loans held for sale	0	(1,471)
Outstanding principal balance, transfers from loans held for investment to loans held for sale	41,934	48,525
Outstanding principal balance, sales	0	0
Valuation adjustment, sales	0	0
Fair value, sales	0	0
Outstanding principal balance, principal payments	(29,640)	(5,214)
Valuation adjustment, principal payments	0	0
Fair value, principal payments	(29,640)	(5,214)
Outstanding principal balance, charge-offs	(4,383)	(4,251)
Valuation adjustment, charge-offs	906	2,169
Fair value, charge-offs	(3,477)	(2,082)
Outstanding principal balance, change in fair value recorded in earnings	0	0
Valuation adjustment, change in fair value recorded in earnings	(2,695)	(809)
Fair value, change in fair value recorded in earnings	(2,695)	(809)
Outstanding principal balance, ending	56,388	47,042	3,518
Valuation adjustment, ending	(6,434)	(3,349)	(935)
Fair value, ending	49,954	43,693	2,583
Loans Held for Sale
Loans
Outstanding principal balance, beginning	0	0
Valuation adjustment, beginning	0	0
Fair value, beginning	0	0
Outstanding principal balance, purchases of loans	1,564,081	2,490,734
Valuation adjustment, purchases of loans	(40,167)	(26,560)
Fair value, purchases of loans	1,523,914	2,464,174
Transfers (to) from loans held for investment and/or loans held for sale	17,413	122,330
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	17,413	122,330
Outstanding principal balance, issuances	0	0
Valuation adjustment, issuances	0	0
Fair value, issuances	0	0
Outstanding principal balance, sales	(1,581,494)	(2,613,064)
Valuation adjustment, sales	52,010	24,789
Fair value, sales	(1,529,484)	(2,588,275)
Outstanding principal balance, principal payments	0	0
Valuation adjustment, principal payments	0	0
Fair value, principal payments	0	0
Outstanding principal balance, charge-offs	0	0
Valuation adjustment, charge-offs	0	0
Fair value, charge-offs	0	0
Outstanding principal balance, change in fair value recorded in earnings	0	0
Valuation adjustment, change in fair value recorded in earnings	(11,843)	1,771
Fair value, change in fair value recorded in earnings	(11,843)	1,771
Outstanding principal balance, ending	0	0	0
Valuation adjustment, ending	0	0	0
Fair value, ending	0	0	0
Loans Held for Sale | Loans Invested in by Company
Loans
Outstanding principal balance, beginning	747,394	869,715
Valuation adjustment, beginning	(25,039)	(29,694)
Fair value, beginning	722,355	840,021
Outstanding principal balance, purchases of loans	1,568,844	5,343,146
Valuation adjustment, purchases of loans	(6)	1
Fair value, purchases of loans	1,568,838	5,343,147
Transfers (to) from loans held for investment and/or loans held for sale	(41,431)	(49,290)
Valuation adjustment, transfers from loans held for investment to loans held for sale	0	1,471
Outstanding principal balance, transfers from loans held for investment to loans held for sale	(41,431)	(47,819)
Outstanding principal balance, sales	(1,907,446)	(5,122,450)
Valuation adjustment, sales	87,723	119,369
Fair value, sales	(1,819,723)	(5,003,081)
Outstanding principal balance, principal payments	(207,483)	(268,366)
Valuation adjustment, principal payments	0	0
Fair value, principal payments	(207,483)	(268,366)
Outstanding principal balance, charge-offs	(27,278)	(25,361)
Valuation adjustment, charge-offs	25,627	23,973
Fair value, charge-offs	(1,651)	(1,388)
Outstanding principal balance, change in fair value recorded in earnings	0	0
Valuation adjustment, change in fair value recorded in earnings	(99,003)	(140,159)
Fair value, change in fair value recorded in earnings	(99,003)	(140,159)
Outstanding principal balance, ending	132,600	747,394	869,715
Valuation adjustment, ending	(10,698)	(25,039)	(29,694)
Fair value, ending	$ 121,902	$ 722,355	$ 840,021