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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	$ 2,829,765	$ 4,295,328	$ 3,141,391	$ 4,547,138
Valuation Adjustment, Beginning	(194,352)	(283,945)	(209,066)	(252,017)
Fair Value, Beginning	2,635,413	4,011,383	2,932,325	4,295,121
Outstanding Principal Balance, Purchases	246,450	472,630	538,564	995,865
Valuation Adjustment, Purchases	0	1	9	3
Fair Value, Purchases	246,450	472,631	538,573	995,868
Outstanding Principal Balance, Transfers from loans to loans held for sale	(1,181)		(1,181)
Valuation Adjustment, Transfers from loans to loans held for sale	(22,152)		(22,152)
Fair Value, Transfers from loans to loans held for sale	(23,333)		(23,333)
Outstanding Principal Balance, Issuances	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	(452,718)	(623,966)	(953,667)	(1,262,201)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(452,718)	(623,966)	(953,667)	(1,262,201)
Outstanding Principal Balance, Charge-offs	(84,060)	(122,376)	(186,851)	(259,186)
Valuation Adjustment, Charge-offs	84,060	122,376	186,851	259,186
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(14,621)	(11,703)	(32,080)	(22,493)
Fair Value, Recoveries	(14,621)	(11,703)	(32,080)	(22,493)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(32,562)	(69,864)	(103,189)	(227,814)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(32,562)	(69,864)	(103,189)	(227,814)
Outstanding Principal Balance, Ending	2,538,256	4,021,616	2,538,256	4,021,616
Valuation Adjustment, Ending	(179,627)	(243,135)	(179,627)	(243,135)
Fair Value, Ending	2,358,629	3,778,481	2,358,629	3,778,481
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	810,558	1,544,595	1,943,835	2,714,802
Valuation Adjustment, Purchases	(1,546)	6,424	(3,318)	6,424
Fair Value, Purchases	809,012	1,551,019	1,940,517	2,721,226
Outstanding Principal Balance, Transfers from loans to loans held for sale	1,181		1,181
Valuation Adjustment, Transfers from loans to loans held for sale	22,152		22,152
Fair Value, Transfers from loans to loans held for sale	23,333		23,333
Outstanding Principal Balance, Issuances	0	0	0	0
Valuation Adjustment, Issuances	0	0	0	0
Fair Value, Issuances	0	0	0	0
Outstanding Principal Balance, Sales	(811,739)	(1,544,595)	(1,945,016)	(2,714,802)
Valuation Adjustment, Sales	773	(6,424)	1,608	(6,424)
Fair Value, Sales	(810,966)	(1,551,019)	(1,943,408)	(2,721,226)
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
Valuation Adjustment, Charge-offs	0	0	0	0
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	0	0	0	0
Fair Value, Recoveries	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(21,379)	0	(20,442)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(21,379)	0	(20,442)	0
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	598,092	26,648	613,652	27,860
Valuation Adjustment, Beginning	(32,290)	(1,867)	(16,597)	(1,949)
Fair Value, Beginning	565,802	24,781	597,055	25,911
Outstanding Principal Balance, Purchases	1,193,071	183,280	1,986,242	190,409
Valuation Adjustment, Purchases	(2,121)	(2)	(2,561)	(9)
Fair Value, Purchases	1,190,950	183,278	1,983,681	190,400
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, Sales	(1,182,533)	(142,355)	(1,925,265)	(146,584)
Valuation Adjustment, Sales	34,831	737	44,879	894
Fair Value, Sales	(1,147,702)	(141,618)	(1,880,386)	(145,690)
Outstanding Principal Balance, Principal payments and retirements	(57,401)	(9,319)	(120,013)	(11,983)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(57,401)	(9,319)	(120,013)	(11,983)
Outstanding Principal Balance, Charge-offs	(4,558)	(1,033)	(7,945)	(2,481)
Valuation Adjustment, Charge-offs	4,558	1,033	7,945	2,481
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(180)	(102)	(391)	(201)
Fair Value, Recoveries	(180)	(102)	(391)	(201)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(26,541)	(2,171)	(55,018)	(3,588)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(26,541)	(2,171)	(55,018)	(3,588)
Outstanding Principal Balance, Ending	546,671	57,221	546,671	57,221
Valuation Adjustment, Ending	(21,743)	(2,372)	(21,743)	(2,372)
Fair Value, Ending		54,849		54,849
Notes and Certificates
Outstanding Principal Balance, Issuances				0
Valuation Adjustment, Issuances				0
Fair Value, Issuances				0
Loans Invested in by Company | Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	339,615	16,924	371,379	18,515
Valuation Adjustment, Beginning	(22,157)	(1,552)	(10,149)	(1,652)
Fair Value, Beginning	317,458	15,372	361,230	16,863
Outstanding Principal Balance, Purchases	1,665	5,620	3,161	6,801
Valuation Adjustment, Purchases	(269)	(2)	(429)	(8)
Fair Value, Purchases	1,396	5,618	2,732	6,793
Outstanding Principal Balance, Transfers from loans to loans held for sale	(316,445)		(316,339)
Valuation Adjustment, Transfers from loans to loans held for sale	22,152		22,152
Fair Value, Transfers from loans to loans held for sale	(294,293)		(294,187)
Outstanding Principal Balance, Issuances				0
Valuation Adjustment, Issuances				0
Fair Value, Issuances				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	(12,120)	(1,778)	(44,314)	(3,262)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(12,120)	(1,778)	(44,314)	(3,262)
Outstanding Principal Balance, Charge-offs	(1,954)	(832)	(3,126)	(2,120)
Valuation Adjustment, Charge-offs	1,954	832	3,126	2,120
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(168)	(102)	(367)	(201)
Fair Value, Recoveries	(168)	(102)	(367)	(201)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(2,652)	(592)	(15,473)	(1,675)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(2,652)	(592)	(15,473)	(1,675)
Outstanding Principal Balance, Ending	10,761	19,934	10,761	19,934
Valuation Adjustment, Ending	(1,140)	(1,416)	(1,140)	(1,416)
Fair Value, Ending	9,621	18,518	9,621	18,518
Loans Invested in by Company | Loans Held For Sale
Loans
Outstanding Principal Balance, Beginning	258,477	9,724	242,273	9,345
Valuation Adjustment, Beginning	(10,133)	(315)	(6,448)	(297)
Fair Value, Beginning	248,344	9,409	235,825	9,048
Outstanding Principal Balance, Purchases	1,191,406	177,660	1,983,081	183,608
Valuation Adjustment, Purchases	(1,852)	0	(2,132)	(1)
Fair Value, Purchases	1,189,554	177,660	1,980,949	183,607
Outstanding Principal Balance, Transfers from loans to loans held for sale	316,445		316,339
Valuation Adjustment, Transfers from loans to loans held for sale	(22,152)		(22,152)
Fair Value, Transfers from loans to loans held for sale	294,293		294,187
Outstanding Principal Balance, Issuances				0
Valuation Adjustment, Issuances				0
Fair Value, Issuances				0
Outstanding Principal Balance, Sales	(1,182,533)	(142,355)	(1,925,265)	(146,584)
Valuation Adjustment, Sales	34,831	737	44,879	894
Fair Value, Sales	(1,147,702)	(141,618)	(1,880,386)	(145,690)
Outstanding Principal Balance, Principal payments and retirements	(45,281)	(7,541)	(75,699)	(8,721)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(45,281)	(7,541)	(75,699)	(8,721)
Outstanding Principal Balance, Charge-offs	(2,604)	(201)	(4,819)	(361)
Valuation Adjustment, Charge-offs	2,604	201	4,819	361
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(12)	0	(24)	0
Fair Value, Recoveries	(12)	0	(24)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(23,889)	(1,579)	(39,545)	(1,913)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(23,889)	(1,579)	(39,545)	(1,913)
Outstanding Principal Balance, Ending	535,910	37,287	535,910	37,287
Valuation Adjustment, Ending	(20,603)	(956)	(20,603)	(956)
Fair Value, Ending	515,307	36,331	515,307	36,331
Notes, Certificates and Secured Borrowings
Notes and Certificates
Outstanding Principal Balance, Beginning	2,847,040	4,318,302	3,161,080	4,572,912
Valuation Adjustment, Beginning	(191,623)	(283,945)	(206,312)	(252,017)
Fair value at beginning of period	2,655,417	4,034,357	2,954,768	4,320,895
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, Issuances	246,517	472,681	538,978	995,986
Valuation Adjustment, Issuances	0	0	0	0
Fair Value, Issuances	246,517	472,681	538,978	995,986
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	(455,571)	(619,889)	(959,281)	(1,260,994)
Valuation Adjustment, Principal payments and retirements	86	0	94	2
Fair Value, Principal payments and retirements	(455,485)	(619,889)	(959,187)	(1,260,992)
Outstanding Principal Balance, Charge-offs	(84,060)	(122,377)	(186,851)	(259,187)
Valuation Adjustment, Charge-offs	84,060	122,377	186,851	259,187
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(14,621)	(11,703)	(32,080)	(22,493)
Fair Value, Recoveries	(14,621)	(11,703)	(32,080)	(22,493)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(54,748)	(69,864)	(125,399)	(227,814)
Fair Value, Change in fair value recorded in earnings	(54,748)	(69,864)	(125,399)	(227,814)
Outstanding Principal Balance, Ending	2,553,926	4,048,717	2,553,926	4,048,717
Valuation Adjustment, Ending	(176,846)	(243,135)	(176,846)	(243,135)
Fair value at end of period	2,377,080	$ 3,805,582	2,377,080	$ 3,805,582
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Principal, Transfers	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Valuation Adjustment, Transfers	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	$ 0		$ 0