Fair Value of Assets and Liabilities - Additional Information about Loans, Notes and Certificates Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Loans Notes And Certificates
Loans
Outstanding Principal Balance, Beginning	$ 4,041,550	$ 4,748,631	$ 4,565,653	$ 4,681,671
Valuation Adjustment, Beginning	(244,551)	(340,870)	(253,669)	(125,590)
Fair Value, Beginning	3,796,999	4,407,761	4,311,984	4,556,081
Outstanding Principal Balance, Purchases	405,472	646,662	1,408,138	2,093,692
Valuation Adjustment, Purchases	(469)	0	(474)	0
Fair Value, Purchases	405,003	646,662	1,407,664	2,093,692
Outstanding Principal Balance, Transfers from loans to loans held for sale	(113,587)	(2,269)	(113,587)	(30,802)
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	0
Fair Value, Transfers from loans to loans held for sale	(113,587)	(2,269)	(113,587)	(30,802)
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	(597,304)	(603,452)	(1,862,767)	(1,782,603)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(597,304)	(603,452)	(1,862,767)	(1,782,603)
Outstanding Principal Balance, Charge-offs	(116,070)	(112,517)	(377,376)	(284,903)
Valuation Adjustment, Charge-offs	116,070	112,517	377,376	284,903
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(12,114)	(10,517)	(34,808)	(27,451)
Fair Value, Recoveries	(12,114)	(10,517)	(34,808)	(27,451)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(65,104)	(26,559)	(294,593)	(397,291)
Fair Value, Change in fair value recorded in earnings	(65,104)	(26,559)	(294,593)	(397,291)
Outstanding Principal Balance, Ending	3,620,061	4,677,055	3,620,061	4,677,055
Valuation Adjustment, Ending	(206,168)	(265,429)	(206,168)	(265,429)
Fair Value, Ending	3,413,893	4,411,626	3,413,893	4,411,626
Notes and Certificates
Outstanding Principal Balance, Beginning	4,048,717	4,755,846	4,572,912	4,697,169
Valuation Adjustment, Beginning	(243,135)	(339,961)	(252,017)	(125,586)
Fair value at beginning of period	3,805,582	4,415,885	4,320,895	4,571,583
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, Transfer from Loans to Loans Held for Sale	0	0	0	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	0	0
Outstanding Principal Balance, Issuances	394,013	644,168	1,389,999	2,047,714
Valuation Adjustment, Issuances	0	0	0	0
Fair Value, Issuances	394,013	644,168	1,389,999	2,047,714
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	(602,051)	(604,160)	(1,863,045)	(1,776,747)
Valuation Adjustment, Principal payments and retirements	2	0	4	0
Fair Value, Principal payments and retirements	(602,049)	(604,160)	(1,863,041)	(1,776,747)
Outstanding Principal Balance, Charge-offs	(115,064)	(111,816)	(374,251)	(284,098)
Valuation Adjustment, Charge-offs	115,064	111,816	374,251	284,098
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(12,057)	(9,955)	(34,550)	(26,871)
Fair Value, Recoveries	(12,057)	(9,955)	(34,550)	(26,871)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(69,911)	(26,027)	(297,725)	(395,768)
Fair Value, Change in fair value recorded in earnings	(69,911)	(26,027)	(297,725)	(395,768)
Outstanding Principal Balance, Ending	3,725,615	4,684,038	3,725,615	4,684,038
Valuation Adjustment, Ending	(210,037)	(264,127)	(210,037)	(264,127)
Fair value at end of period	3,515,578	4,419,911	3,515,578	4,419,911
Loans Held For Sale(1)(2)
Loans
Outstanding Principal Balance, Beginning	37,287	16,627	9,345	0
Valuation Adjustment, Beginning	(956)	(217)	(297)	0
Fair Value, Beginning	36,331	16,410	9,048	0
Outstanding Principal Balance, Purchases	1,931,088	1,093,868	4,829,498	3,623,453
Valuation Adjustment, Purchases	(2,136)	0	4,287	0
Fair Value, Purchases	1,928,952	1,093,868	4,833,785	3,623,453
Outstanding Principal Balance, Transfers from loans to loans held for sale	113,587	2,269	113,587	30,802
Valuation Adjustment, Transfers from loans to loans held for sale	0	0	0	0
Fair Value, Transfers from loans to loans held for sale	113,587	2,269	113,587	30,802
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	(1,787,527)	(1,095,716)	(4,648,913)	(3,635,330)
Valuation Adjustment, Sales	3,874	0	(1,656)	0
Fair Value, Sales	(1,783,653)	(1,095,716)	(4,650,569)	(3,635,330)
Outstanding Principal Balance, Principal payments and retirements	(16,710)	(2,143)	(25,431)	(4,020)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(16,710)	(2,143)	(25,431)	(4,020)
Outstanding Principal Balance, Charge-offs	(534)	0	(895)	0
Valuation Adjustment, Charge-offs	534	0	895	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
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(12,582)	56	(14,495)	(161)
Fair Value, Change in fair value recorded in earnings	(12,582)	56	(14,495)	(161)
Outstanding Principal Balance, Ending	277,191	14,905	277,191	14,905
Valuation Adjustment, Ending	(11,266)	(161)	(11,266)	(161)
Fair Value, Ending	265,925	14,744	265,925	14,744
Loans Invested in by Company
Loans
Outstanding Principal Balance, Beginning	57,221	35,672	27,860	3,462
Valuation Adjustment, Beginning	(2,371)	(1,126)	(1,949)	(4)
Fair Value, Beginning	54,850	34,546	25,911	3,458
Outstanding Principal Balance, Purchases	586,708	3,579	777,117	175,600
Valuation Adjustment, Purchases	(473)	0	(481)	0
Fair Value, Purchases	586,235	3,579	776,636	175,600
Outstanding Principal Balance, Sales	(429,481)	(6)	(576,065)	(135,531)
Valuation Adjustment, Sales	1,741	0	2,634	0
Fair Value, Sales	(427,740)	(6)	(573,431)	(135,531)
Outstanding Principal Balance, Principal payments and retirements	(18,720)	(4,352)	(30,703)	(8,534)
Valuation Adjustment, Principal payments	0	0	0	0
Fair Value, Principal payments and retirements	(18,720)	(4,352)	(30,703)	(8,534)
Outstanding Principal Balance, Charge-offs	(1,538)	(701)	(4,019)	(805)
Valuation Adjustment, Charge-offs	1,538	701	4,019	805
Fair Value, Charge-offs	0	0	0	0
Outstanding Principal Balance, Recoveries	0	0	0	0
Valuation Adjustment, Recoveries	(56)	(562)	(256)	(580)
Fair Value, Recoveries	(56)	(562)	(256)	(580)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	(7,775)	(476)	(11,363)	(1,684)
Fair Value, Change in fair value recorded in earnings	(7,775)	(476)	(11,363)	(1,684)
Outstanding Principal Balance, Ending	194,190	34,192	194,190	34,192
Valuation Adjustment, Ending	(7,396)	(1,463)	(7,396)	(1,463)
Fair Value, Ending	$ 186,794	$ 32,729	$ 186,794	$ 32,729