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	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	$ 990,662	$ 1,805,078	$ 1,148,888	$ 2,013,438
Valuation Adjustment, Beginning	(105,249)	(106,880)	(69,573)	(130,187)
Fair Value, Beginning	885,413	1,698,198	1,079,315	1,883,251
Outstanding Principal Balance, Purchases	55,274	166,754	159,894	359,846
Valuation Adjustment, Purchases	0	0	0	(21)
Fair Value, Purchases	55,274	166,754	159,894	359,825
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale	(101)	(229)	(17,579)	(452)
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	0
Fair Value, Transfers from loans to loans held for sale	(101)	(229)	(17,579)	(452)
Outstanding Principal Balance, Issuance	0	0	0	0
Valuation Adjustment, Issuances	0	0	0	0
Fair Value, Issuances	0	0	0	0
Outstanding Principal Balance, Sales	0	0	0	0
Valuation Adjustment, Sales	0	0	0	0
Fair Value, Sales	0	0	0	0
Outstanding Principal Balance, Principal payments and retirements	(171,541)	(317,919)	(387,289)	(658,363)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(171,541)	(317,919)	(387,289)	(658,363)
Outstanding Principal Balance, Charge-offs	(22,818)	(48,427)	(52,438)	(109,212)
Charge-offs, net of recoveries	14,202	34,800	31,513	81,676
Fair Value, Charge-offs	(8,616)	(13,627)	(20,925)	(27,536)
Change in fair value recorded in earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	24,799	(20,193)	(28,188)	(43,741)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	24,799	(20,193)	(28,188)	(43,741)
Outstanding Principal Balance, Ending	851,476	1,605,257	851,476	1,605,257
Valuation Adjustment, Ending	(66,248)	(92,273)	(66,248)	(92,273)
Fair Value, Ending	785,228	1,512,984	785,228	1,512,984
Loans Held For Sale
Loans
Outstanding Principal Balance, Beginning	0	0	0	0
Valuation Adjustment, Beginning	0	0	0	0
Fair Value, Beginning	0	0	0	0
Outstanding Principal Balance, Purchases	57,760	552,115	522,588	1,115,151
Valuation Adjustment, Purchases	(1,044)	(468)	(2,923)	(468)
Fair Value, Purchases	56,716	551,647	519,665	1,114,683
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale	0	0	17,413	0
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	0
Fair Value, Transfers from loans to loans held for sale	0	0	17,413	0
Outstanding Principal Balance, Issuance	0	0	0	0
Valuation Adjustment, Issuances	0	0	0	0
Fair Value, Issuances	0	0	0	0
Outstanding Principal Balance, Sales	(57,760)	(552,115)	(540,001)	(1,115,151)
Valuation Adjustment, Sales	1,224	385	3,463	(780)
Fair Value, Sales	(56,536)	(551,730)	(536,538)	(1,115,931)
Outstanding Principal Balance, Principal payments and retirements	0	0	0	0
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	0	0	0	0
Outstanding Principal Balance, Charge-offs	0	0	0	0
Charge-offs, net of recoveries	0	0	0	0
Fair Value, Charge-offs	0	0	0	0
Change in fair value recorded in earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(180)	83	(540)	1,248
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(180)	83	(540)	1,248
Outstanding Principal Balance, Ending	0	0	0	0
Valuation Adjustment, Ending	0	0	0	0
Fair Value, Ending	0	0	0	0
Loans Invested in by Company
Loans
Outstanding Principal Balance, Beginning	919,207	596,105	794,436	873,233
Valuation Adjustment, Beginning	(106,500)	(35,182)	(28,388)	(30,629)
Fair Value, Beginning	812,707	560,923	766,048	842,604
Outstanding Principal Balance, Purchases	23,749	1,189,792	1,403,570	2,033,721
Valuation Adjustment, Purchases	(312)	(615)	(1,005)	(994)
Fair Value, Purchases	23,437	1,189,177	1,402,565	2,032,727
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale	101	229	166	452
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	0
Fair Value, Transfers from loans to loans held for sale	101	229	166	452
Outstanding Principal Balance, Sales	(112,527)	(1,249,866)	(1,283,934)	(2,295,746)
Valuation Adjustment, Sales	10,947	40,544	30,856	62,294
Fair Value, Sales	(101,580)	(1,209,322)	(1,253,078)	(2,233,452)
Outstanding Principal Balance, Principal payments and retirements	(79,537)	(64,980)	(155,324)	(133,333)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(79,537)	(64,980)	(155,324)	(133,333)
Outstanding Principal Balance, Charge-offs	(7,572)	(6,943)	(15,493)	(13,990)
Charge-offs, net of recoveries	6,395	6,193	12,308	12,810
Fair Value, Charge-offs	(1,177)	(750)	(3,185)	(1,180)
Change in fair value recorded in earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(1,301)	(35,167)	(104,542)	(67,708)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(1,301)	(35,167)	(104,542)	(67,708)
Outstanding Principal Balance, Ending	743,421	464,337	743,421	464,337
Valuation Adjustment, Ending	(90,771)	(24,227)	(90,771)	(24,227)
Fair Value, Ending		440,110		440,110
Loans Invested in by Company | Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	83,650	11,233	47,042	3,518
Valuation Adjustment, Beginning	(12,647)	(2,476)	(3,349)	(935)
Fair Value, Beginning	71,003	8,757	43,693	2,583
Outstanding Principal Balance, Purchases	386	789	1,113	1,170
Valuation Adjustment, Purchases	(312)	(615)	(1,005)	(994)
Fair Value, Purchases	74	174	108	176
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale	137	(3,583)	43,325	4,950
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	(1,471)
Fair Value, Transfers from loans to loans held for sale	137	(3,583)	43,325	3,479
Outstanding Principal Balance, Sales	0	0	0	0
Valuation Adjustment, Sales	0	0	0	0
Fair Value, Sales	0	0	0	0
Outstanding Principal Balance, Principal payments and retirements	(7,949)	(920)	(13,633)	(1,455)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(7,949)	(920)	(13,633)	(1,455)
Outstanding Principal Balance, Charge-offs	(1,065)	(969)	(2,688)	(1,633)
Charge-offs, net of recoveries	377	410	511	847
Fair Value, Charge-offs	(688)	(559)	(2,177)	(786)
Change in fair value recorded in earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	2,980	1,158	(5,759)	1,030
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	2,980	1,158	(5,759)	1,030
Outstanding Principal Balance, Ending	75,159	6,550	75,159	6,550
Valuation Adjustment, Ending	(9,602)	(1,523)	(9,602)	(1,523)
Fair Value, Ending	65,557	5,027	65,557	5,027
Loans Invested in by Company | Loans Held For Sale
Loans
Outstanding Principal Balance, Beginning	835,557	584,872	747,394	869,715
Valuation Adjustment, Beginning	(93,853)	(32,706)	(25,039)	(29,694)
Fair Value, Beginning	741,704	552,166	722,355	840,021
Outstanding Principal Balance, Purchases	23,363	1,189,003	1,402,457	2,032,551
Valuation Adjustment, Purchases	0	0	0	0
Fair Value, Purchases	23,363	1,189,003	1,402,457	2,032,551
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale	(36)	3,812	(43,159)	(4,498)
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	1,471
Fair Value, Transfers from loans to loans held for sale	(36)	3,812	(43,159)	(3,027)
Outstanding Principal Balance, Sales	(112,527)	(1,249,866)	(1,283,934)	(2,295,746)
Valuation Adjustment, Sales	10,947	40,544	30,856	62,294
Fair Value, Sales	(101,580)	(1,209,322)	(1,253,078)	(2,233,452)
Outstanding Principal Balance, Principal payments and retirements	(71,588)	(64,060)	(141,691)	(131,878)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(71,588)	(64,060)	(141,691)	(131,878)
Outstanding Principal Balance, Charge-offs	(6,507)	(5,974)	(12,805)	(12,357)
Charge-offs, net of recoveries	6,018	5,783	11,797	11,963
Fair Value, Charge-offs	(489)	(191)	(1,008)	(394)
Change in fair value recorded in earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(4,281)	(36,325)	(98,783)	(68,738)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(4,281)	(36,325)	(98,783)	(68,738)
Outstanding Principal Balance, Ending	668,262	457,787	668,262	457,787
Valuation Adjustment, Ending	(81,169)	(22,704)	(81,169)	(22,704)
Fair Value, Ending	587,093	435,083	587,093	435,083
Notes, Certificates and Secured Borrowings
Notes and Certificates
Outstanding Principal Balance, Beginning	990,662	1,805,078	1,148,888	2,033,258
Valuation Adjustment, Beginning	(103,822)	(101,852)	(67,422)	(127,383)
Fair value at beginning of period	886,840	1,703,226	1,081,466	1,905,875
Outstanding Principal Balance, Purchases	0	0	0	0
Valuation Adjustment, Purchases	0	0	0	0
Fair Value, Purchases	0	0	0	0
Outstanding Principal Balance, Beginning, Transfers	0	0	0	0
Valuation Adjustment, Transfers	0	0	0	0
Fair Value, Transfers	0	0	0	0
Outstanding Principal Balance, Issuances	55,274	166,754	159,894	359,846
Valuation Adjustment, Issuances	0	0	0	0
Fair Value, Issuances	55,274	166,754	159,894	359,846
Outstanding Principal Balance, Sales	0	0	0	0
Valuation Adjustment, Sales	0	0	0	0
Fair Value, Sales	0	0	0	0
Outstanding Principal Balance, Principal payments and retirements	(171,642)	(318,148)	(404,868)	(678,635)
Valuation Adjustment, Principal payments and retirements	0	0	0	14
Fair Value, Principal payments and retirements	(171,642)	(318,148)	(404,868)	(678,621)
Outstanding Principal Balance, Charge-offs	(22,818)	(48,427)	(52,438)	(109,212)
Valuation Adjustment, Charge-offs	13,556	34,789	29,987	81,665
Fair Value, Charge-offs	(9,262)	(13,638)	(22,451)	(27,547)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	24,718	(20,243)	(28,113)	(41,602)
Fair Value, Change in fair value recorded in earnings	24,718	(20,243)	(28,113)	(41,602)
Outstanding Principal Balance, Ending	851,476	1,605,257	851,476	1,605,257
Valuation Adjustment, Ending	(65,548)	(87,306)	(65,548)	(87,306)
Fair value at end of period	785,928	1,517,951	785,928	1,517,951
Fair Value, Measurements, Recurring | Loans Invested in by Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable and Loans Held for Sale, Fair Value Disclosure	$ 652,650	$ 440,110	$ 652,650	$ 440,110	$ 766,048