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	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total Loans Invested in by Company
Loans
Outstanding Principal Balance, Beginning	$ 873,233	$ 873,233	$ 613,652
Valuation adjustment, Beginning	(30,629)	(30,629)	(16,597)
Fair Value, Beginning	842,604	842,604	597,055
Outstanding Principal Balance, Purchases of loans	5,300,000	5,346,139	4,362,155
Valuation Adjustment, Purchases of loans		(2,302)	(3,615)
Fair Value, Purchases of loans		5,343,837	4,358,540
Transfers (to) from loans held for investment and/or loans held for sale		706	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		706	0
Outstanding Principal Balance, Sales		(5,122,450)	(3,862,910)
Valuation Adjustment, Sales		119,369	72,742
Fair Value, Sales		(5,003,081)	(3,790,168)
Outstanding Principal Balance, Principal payments		(273,580)	(219,886)
Valuation Adjustment, Principal payments		0	0
Fair Value, Principal payments		(273,580)	(219,886)
Outstanding Principal Balance, Charge-offs		(29,612)	(19,778)
Valuation Adjustment, Charge-offs		26,142	18,856
Fair Value, Charge-offs		(3,470)	(922)
Outstanding Principal Balance, Change in fair value recorded in earnings		0	0
Valuation Adjustment, Change in fair value recorded in earnings	(141,000)	(140,968)	(102,015)	$ (25,800)
Fair Value, Change in fair value recorded in earnings		(140,968)	(102,015)
Outstanding Principal Balance, Ending		794,436	873,233	613,652
Valuation adjustment, Ending		(28,388)	(30,629)	(16,597)
Fair Value, Ending		766,048	842,604	597,055
Notes, Certificates and Secured Borrowings
Notes, Certificates and Secured Borrowings
Outstanding Principal Balance, Beginning	2,033,258	2,033,258	3,161,080
Valuation adjustment, Beginning	(127,383)	(127,383)	(206,312)
Fair value at beginning of period	1,905,875	1,905,875	2,954,768
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		632,962	953,904
Valuation Adjustment, Issuances of notes and certificates		0	0
Fair Value, Issuances of notes and certificates		632,962	953,904
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		(1,326,526)	(1,756,212)
Valuation Adjustment, Whole loan sales		14	111
Fair Value, Whole loan sales		(1,326,512)	(1,756,101)
Outstanding Principal Balance, Charge-offs		(190,806)	(325,514)
Valuation Adjustment, Charge-offs		135,785	263,020
Fair Value, Charge-offs		(55,021)	(62,494)
Outstanding Principal Balance, Change in fair value recorded in earnings		0	0
Valuation Adjustment, Change in fair value recorded in earnings		(75,838)	(184,202)
Fair Value, Change in fair value recorded in earnings		(75,838)	(184,202)
Outstanding Principal Balance, Ending		1,148,888	2,033,258	3,161,080
Valuation adjustment, Ending		(67,422)	(127,383)	(206,312)
Fair value at end of period		1,081,466	1,905,875	2,954,768
Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	2,013,438	2,013,438	3,141,391
Valuation adjustment, Beginning	(130,187)	(130,187)	(209,066)
Fair Value, Beginning	1,883,251	1,883,251	2,932,325
Outstanding Principal Balance, Purchases of loans		632,962	953,034
Valuation Adjustment, Purchases of loans		(21)	26
Fair Value, Purchases of loans		632,941	953,060
Transfers (to) from loans held for investment and/or loans held for sale		(123,036)	(1,180)
Valuation Adjustment, Transfers from loans held for investment to loans held for sale		0	(22,152)
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale		(123,036)	(23,332)
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		(1,183,670)	(1,754,293)
Valuation Adjustment, Principal payments		0	0
Fair Value, Principal payments		(1,183,670)	(1,754,293)
Outstanding Principal Balance, Charge-offs		(190,806)	(325,514)
Valuation Adjustment, Charge-offs		138,857	263,022
Fair Value, Charge-offs		(51,949)	(62,492)
Outstanding Principal Balance, Change in fair value recorded in earnings		0	0
Valuation Adjustment, Change in fair value recorded in earnings		(78,222)	(162,017)
Fair Value, Change in fair value recorded in earnings		(78,222)	(162,017)
Outstanding Principal Balance, Ending		1,148,888	2,013,438	3,141,391
Valuation adjustment, Ending		(69,573)	(130,187)	(209,066)
Fair Value, Ending		1,079,315	1,883,251	2,932,325
Loans Held for Investment | Total Loans Invested in by Company
Loans
Outstanding Principal Balance, Beginning	3,518	3,518	371,379
Valuation adjustment, Beginning	(935)	(935)	(10,149)
Fair Value, Beginning	2,583	2,583	361,230
Outstanding Principal Balance, Purchases of loans		2,993	8,697
Valuation Adjustment, Purchases of loans		(2,303)	(876)
Fair Value, Purchases of loans		690	7,821
Transfers (to) from loans held for investment and/or loans held for sale		49,996	(324,626)
Valuation Adjustment, Transfers from loans held for investment to loans held for sale		(1,471)	22,152
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale		48,525	(302,474)
Outstanding Principal Balance, Sales		0	0
Valuation Adjustment, Sales		0	0
Fair Value, Sales		0	0
Outstanding Principal Balance, Principal payments		(5,214)	(47,552)
Valuation Adjustment, Principal payments		0	0
Fair Value, Principal payments		(5,214)	(47,552)
Outstanding Principal Balance, Charge-offs		(4,251)	(4,380)
Valuation Adjustment, Charge-offs		2,169	3,633
Fair Value, Charge-offs		(2,082)	(747)
Outstanding Principal Balance, Change in fair value recorded in earnings		0	0
Valuation Adjustment, Change in fair value recorded in earnings		(809)	(15,695)
Fair Value, Change in fair value recorded in earnings		(809)	(15,695)
Outstanding Principal Balance, Ending		47,042	3,518	371,379
Valuation adjustment, Ending		(3,349)	(935)	(10,149)
Fair Value, Ending		43,693	2,583	361,230
Loans Held for Sale
Loans
Outstanding Principal Balance, Beginning	0	0	0
Valuation adjustment, Beginning	0	0	0
Fair Value, Beginning	0	0	0
Outstanding Principal Balance, Purchases of loans		2,490,734	3,141,891
Valuation Adjustment, Purchases of loans		(26,560)	(5,714)
Fair Value, Purchases of loans		2,464,174	3,136,177
Transfers (to) from loans held for investment and/or loans held for sale		122,330	1,180
Valuation Adjustment, Transfers from loans held for investment to loans held for sale		0	22,152
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale		122,330	23,332
Outstanding Principal Balance, Issuances		0	0
Valuation Adjustment, Issuances		0	0
Fair Value, Issuances		0	0
Outstanding Principal Balance, Sales		(2,613,064)	(3,143,071)
Valuation Adjustment, Sales		24,789	1,548
Fair Value, Sales		(2,588,275)	(3,141,523)
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		1,771	(17,986)
Fair Value, Change in fair value recorded in earnings		1,771	(17,986)
Outstanding Principal Balance, Ending		0	0	0
Valuation adjustment, Ending		0	0	0
Fair Value, Ending		0	0	0
Loans Held for Sale | Total Loans Invested in by Company
Loans
Outstanding Principal Balance, Beginning	869,715	869,715	242,273
Valuation adjustment, Beginning	(29,694)	(29,694)	(6,448)
Fair Value, Beginning	$ 840,021	840,021	235,825
Outstanding Principal Balance, Purchases of loans		5,343,146	4,353,458
Valuation Adjustment, Purchases of loans		1	(2,739)
Fair Value, Purchases of loans		5,343,147	4,350,719
Transfers (to) from loans held for investment and/or loans held for sale		(49,290)	324,626
Valuation Adjustment, Transfers from loans held for investment to loans held for sale		1,471	(22,152)
Outstanding Principal Balance, Transfers from loans held for investment to loans held for sale		(47,819)	302,474
Outstanding Principal Balance, Sales		(5,122,450)	(3,862,910)
Valuation Adjustment, Sales		119,369	72,742
Fair Value, Sales		(5,003,081)	(3,790,168)
Outstanding Principal Balance, Principal payments		(268,366)	(172,334)
Valuation Adjustment, Principal payments		0	0
Fair Value, Principal payments		(268,366)	(172,334)
Outstanding Principal Balance, Charge-offs		(25,361)	(15,398)
Valuation Adjustment, Charge-offs		23,973	15,223
Fair Value, Charge-offs		(1,388)	(175)
Outstanding Principal Balance, Change in fair value recorded in earnings		0	0
Valuation Adjustment, Change in fair value recorded in earnings		(140,159)	(86,320)
Fair Value, Change in fair value recorded in earnings		(140,159)	(86,320)
Outstanding Principal Balance, Ending		747,394	869,715	242,273
Valuation adjustment, Ending		(25,039)	(29,694)	(6,448)
Fair Value, Ending		$ 722,355	$ 840,021	$ 235,825