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
	Mar. 31, 2018	Mar. 31, 2017
Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	$ 3,141,391	$ 4,547,138
Valuation Adjustment, Beginning	(209,066)	(252,017)
Fair Value, Beginning	2,932,325	4,295,121
Outstanding Principal Balance, Purchases	292,114	523,235
Valuation Adjustment, Purchases	9	2
Fair Value, Purchases	292,123	523,237
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 and retirements	(500,949)	(638,235)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments and retirements	(500,949)	(638,235)
Outstanding Principal Balance, Charge-offs	(102,791)	(136,810)
Valuation Adjustment, Charge-offs	102,791	136,810
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(17,459)	(10,790)
Fair Value, Recoveries	(17,459)	(10,790)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(70,627)	(157,950)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(70,627)	(157,950)
Outstanding Principal Balance, Ending	2,829,765	4,295,328
Valuation Adjustment, Ending	(194,352)	(283,945)
Fair Value, Ending	2,635,413	4,011,383
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	1,133,277	1,170,207
Valuation Adjustment, Purchases	(1,772)	0
Fair Value, Purchases	1,131,505	1,170,207
Outstanding Principal Balance, Issuances	0	0
Valuation Adjustment, Issuances	0	0
Fair Value, Issuances	0	0
Outstanding Principal Balance, Sales	(1,133,277)	(1,170,207)
Valuation Adjustment, Sales	835	0
Fair Value, Sales	(1,132,442)	(1,170,207)
Outstanding Principal Balance, Principal payments and retirements	0	0
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments and retirements	0	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
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	937	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	937	0
Outstanding Principal Balance, Ending	0	0
Valuation Adjustment, Ending	0	0
Fair Value, Ending	0	0
Notes, Certificates and Secured Borrowings
Notes and Certificates
Outstanding Principal Balance, Beginning	3,161,080	4,572,912
Valuation Adjustment, Beginning	(206,312)	(252,017)
Fair value at beginning of period	2,954,768	4,320,895
Outstanding Principal Balance, Purchases	0	0
Valuation Adjustment, Purchases	0	0
Fair Value, Purchases	0	0
Outstanding Principal Balance, Issuances	292,461	523,305
Valuation Adjustment, Issuances	0	0
Fair Value, Issuances	292,461	523,305
Outstanding Principal Balance, Sales	0	0
Valuation Adjustment, Sales	0	0
Fair Value, Sales	0	0
Outstanding Principal Balance, Principal payments and retirements	(503,710)	(641,105)
Valuation Adjustment, Principal payments and retirements	8	2
Fair Value, Principal payments and retirements	(503,702)	(641,103)
Outstanding Principal Balance, Charge-offs	(102,791)	(136,810)
Valuation Adjustment, Charge-offs	102,791	136,810
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(17,459)	(10,790)
Fair Value, Recoveries	(17,459)	(10,790)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(70,651)	(157,950)
Fair Value, Change in fair value recorded in earnings	(70,651)	(157,950)
Outstanding Principal Balance, Ending	2,847,040	4,318,302
Valuation Adjustment, Ending	(191,623)	(283,945)
Fair value at end of period	2,655,417	4,034,357
Loans Invested in by Company
Loans
Outstanding Principal Balance, Beginning	613,652	27,860
Valuation Adjustment, Beginning	(16,597)	(1,949)
Fair Value, Beginning	597,055	25,911
Outstanding Principal Balance, Purchases	793,171	7,129
Valuation Adjustment, Purchases	(440)	(7)
Fair Value, Purchases	792,731	7,122
Outstanding Principal Balance, Transfers from loans to loans held for sale	0
Valuation Adjustment, Transfers from loans to loans held for sale	0
Fair Value, Transfers from loans to loans held for sale	0
Outstanding Principal Balance, Sales	(742,732)	(4,229)
Valuation Adjustment, Sales	10,048	157
Fair Value, Sales	(732,684)	(4,072)
Outstanding Principal Balance, Principal payments and retirements	(62,612)	(2,664)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments and retirements	(62,612)	(2,664)
Outstanding Principal Balance, Charge-offs	(3,387)	(1,448)
Valuation Adjustment, Charge-offs	3,387	1,448
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(211)	(99)
Fair Value, Recoveries	(211)	(99)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(28,477)	(1,417)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(28,477)	(1,417)
Outstanding Principal Balance, Ending	598,092	26,648
Valuation Adjustment, Ending	(32,290)	(1,867)
Fair Value, Ending		24,781
Loans Invested in by Company | Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	371,379	18,515
Valuation Adjustment, Beginning	(10,149)	(1,652)
Fair Value, Beginning	361,230	16,863
Outstanding Principal Balance, Purchases	1,496	1,181
Valuation Adjustment, Purchases	(160)	(6)
Fair Value, Purchases	1,336	1,175
Outstanding Principal Balance, Transfers from loans to loans held for sale	106
Valuation Adjustment, Transfers from loans to loans held for sale	0
Fair Value, Transfers from loans to loans held for sale	106
Outstanding Principal Balance, Sales	0	0
Valuation Adjustment, Sales	0	0
Fair Value, Sales	0	0
Outstanding Principal Balance, Principal payments and retirements	(32,194)	(1,484)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments and retirements	(32,194)	(1,484)
Outstanding Principal Balance, Charge-offs	(1,172)	(1,288)
Valuation Adjustment, Charge-offs	1,172	1,288
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(199)	(99)
Fair Value, Recoveries	(199)	(99)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(12,821)	(1,083)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(12,821)	(1,083)
Outstanding Principal Balance, Ending	339,615	16,924
Valuation Adjustment, Ending	(22,157)	(1,552)
Fair Value, Ending	317,458	15,372
Loans Invested in by Company | Loans Held For Sale
Loans
Outstanding Principal Balance, Beginning	242,273	9,345
Valuation Adjustment, Beginning	(6,448)	(297)
Fair Value, Beginning	235,825	9,048
Outstanding Principal Balance, Purchases	791,675	5,948
Valuation Adjustment, Purchases	(280)	(1)
Fair Value, Purchases	791,395	5,947
Outstanding Principal Balance, Transfers from loans to loans held for sale	(106)
Valuation Adjustment, Transfers from loans to loans held for sale	0
Fair Value, Transfers from loans to loans held for sale	(106)
Outstanding Principal Balance, Sales	(742,732)	(4,229)
Valuation Adjustment, Sales	10,048	157
Fair Value, Sales	(732,684)	(4,072)
Outstanding Principal Balance, Principal payments and retirements	(30,418)	(1,180)
Valuation Adjustment, Principal payments	0	0
Fair Value, Principal payments and retirements	(30,418)	(1,180)
Outstanding Principal Balance, Charge-offs	(2,215)	(160)
Valuation Adjustment, Charge-offs	2,215	160
Fair Value, Charge-offs	0	0
Outstanding Principal Balance, Recoveries	0	0
Valuation Adjustment, Recoveries	(12)	0
Fair Value, Recoveries	(12)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Principal, Gain (Loss) Included in Earnings	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Valuation Adjustment, Gain (Loss) Included in Earnings	(15,656)	(334)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(15,656)	(334)
Outstanding Principal Balance, Ending	258,477	9,724
Valuation Adjustment, Ending	(10,133)	(315)
Fair Value, Ending	$ 248,344	$ 9,409