Fair Value of Assets and Liabilities - Fair Value Reconciliation (Detail) - Loans Invested in by Company - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Loans Held For Sale
Loans
Outstanding Principal Balance, Beginning	$ 186,988	$ 668,262	$ 132,600	$ 747,394
Valuation Adjustment, Beginning	(6,927)	(81,169)	(10,698)	(25,039)
Fair Value, Beginning	180,061	587,093	121,902	722,355
Outstanding Principal Balance, Purchases	2,339,869	27,756	5,320,264	1,430,213
Valuation Adjustment, Purchases	0	(6)	(1,629)	(6)
Fair Value, Purchases	2,339,869	27,750	5,318,635	1,430,207
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale		479	1,181	(42,680)
Valuation Adjustment, Transfers (to) from loans held for investment and/or loans held for sale		0	0	0
Fair Value, Transfers (to) from loans held for investment and/or loans held for sale		479	1,181	(42,680)
Outstanding Principal Balance, Sales	(2,342,158)	(442,744)	(5,211,984)	(1,726,678)
Valuation Adjustment, Sales	(85)	51,220	10,568	82,076
Fair Value, Sales	(2,342,243)	(391,524)	(5,201,416)	(1,644,602)
Outstanding Principal Balance, Principal payments and retirements	(27,211)	(44,439)	(78,946)	(186,130)
Valuation Adjustment, Principal payments and retirements	0	0	0	0
Fair Value, Principal payments and retirements	(27,211)	(44,439)	(78,946)	(186,130)
Outstanding Principal Balance, Charge-offs, net of recoveries	(1,138)	(6,565)	(6,765)	(19,370)
Valuation Adjustment, Charge-offs, net of recoveries	(209)	6,148	3,711	17,945
Fair Value, Charge-offs, net of recoveries	(1,347)	(417)	(3,054)	(1,425)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	3,031	1,859	(6,142)	(96,924)
Fair Value, Change in fair value recorded in earnings	3,031	1,859	(6,142)	(96,924)
Outstanding Principal Balance, Ending	156,350	202,749	156,350	202,749
Valuation Adjustment, Ending	(4,190)	(21,948)	(4,190)	(21,948)
Fair Value, Ending	152,160	180,801	152,160	180,801
Loans Held for Investment
Loans
Outstanding Principal Balance, Beginning	413,935	851,476	679,903	1,148,888
Valuation Adjustment, Beginning	(22,573)	(66,248)	(43,217)	(69,573)
Fair Value, Beginning	391,362	785,228	636,686	1,079,315
Outstanding Principal Balance, Purchases		77,389		237,283
Valuation Adjustment, Purchases		0		0
Fair Value, Purchases		77,389		237,283
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale		(324)		(17,903)
Valuation Adjustment, Transfers (to) from loans held for investment and/or loans held for sale		0		0
Fair Value, Transfers (to) from loans held for investment and/or loans held for sale		(324)		(17,903)
Outstanding Principal Balance, Issuance		0		0
Valuation Adjustment, Issuances		0		0
Fair Value, Issuances		0		0
Outstanding Principal Balance, Principal payments and retirements	(94,152)	(159,753)	(339,911)	(547,042)
Valuation Adjustment, Principal payments and retirements	0	0	0	0
Fair Value, Principal payments and retirements	(94,152)	(159,753)	(339,911)	(547,042)
Outstanding Principal Balance, Charge-offs, net of recoveries	(5,775)	(12,218)	(25,984)	(64,656)
Valuation Adjustment, Charge-offs, net of recoveries	(3,293)	2,049	3,890	33,562
Fair Value, Charge-offs, net of recoveries	(9,068)	(10,169)	(22,094)	(31,094)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	10,399	15,903	23,860	(12,285)
Fair Value, Change in fair value recorded in earnings	10,399	15,903	23,860	(12,285)
Outstanding Principal Balance, Ending	314,008	756,570	314,008	756,570
Valuation Adjustment, Ending	(15,467)	(48,296)	(15,467)	(48,296)
Fair Value, Ending	298,541	708,274	298,541	708,274
Notes, Certificates and Secured Borrowings
Loans
Outstanding Principal Balance, Beginning	413,935	851,476	679,903	1,148,888
Valuation Adjustment, Beginning	(22,551)	(65,548)	(43,129)	(67,422)
Fair Value, Beginning	391,384	785,928	636,774	1,081,466
Outstanding Principal Balance, Purchases		0		0
Valuation Adjustment, Purchases		0		0
Fair Value, Purchases		0		0
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale		0		0
Valuation Adjustment, Transfers (to) from loans held for investment and/or loans held for sale		0		0
Fair Value, Transfers (to) from loans held for investment and/or loans held for sale		0		0
Outstanding Principal Balance, Issuance		77,389		237,283
Valuation Adjustment, Issuances		0		0
Fair Value, Issuances		77,389		237,283
Outstanding Principal Balance, Principal payments and retirements	(94,152)	(160,077)	(339,911)	(564,945)
Valuation Adjustment, Principal payments and retirements	0	0	0	0
Fair Value, Principal payments and retirements	(94,152)	(160,077)	(339,911)	(564,945)
Outstanding Principal Balance, Charge-offs, net of recoveries	(5,775)	(12,218)	(25,984)	(64,656)
Valuation Adjustment, Charge-offs, net of recoveries	(3,315)	1,358	3,735	31,345
Fair Value, Charge-offs, net of recoveries	(9,090)	(10,860)	(22,249)	(33,311)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	10,399	16,217	23,927	(11,896)
Fair Value, Change in fair value recorded in earnings	10,399	16,217	23,927	(11,896)
Outstanding Principal Balance, Ending	314,008	756,570	314,008	756,570
Valuation Adjustment, Ending	(15,467)	(47,973)	(15,467)	(47,973)
Fair Value, Ending	298,541	708,597	298,541	708,597
Other Loans Held For Investment
Loans
Outstanding Principal Balance, Beginning	38,137	75,159	56,388	47,042
Valuation Adjustment, Beginning	(3,614)	(9,602)	(6,434)	(3,349)
Fair Value, Beginning	34,523	65,557	49,954	43,693
Outstanding Principal Balance, Purchases	6	201	161	1,314
Valuation Adjustment, Purchases	2	(189)	(69)	(1,194)
Fair Value, Purchases	8	12	92	120
Outstanding Principal Balance, Transfers (to) from loans held for investment and/or loans held for sale		(155)		43,170
Valuation Adjustment, Transfers (to) from loans held for investment and/or loans held for sale		0		0
Fair Value, Transfers (to) from loans held for investment and/or loans held for sale		(155)		43,170
Outstanding Principal Balance, Principal payments and retirements	(7,654)	(8,205)	(24,224)	(21,838)
Valuation Adjustment, Principal payments and retirements	0	0	0	0
Fair Value, Principal payments and retirements	(7,654)	(8,205)	(24,224)	(21,838)
Outstanding Principal Balance, Charge-offs, net of recoveries	(395)	(768)	(2,231)	(3,456)
Valuation Adjustment, Charge-offs, net of recoveries	324	245	1,653	756
Fair Value, Charge-offs, net of recoveries	(71)	(523)	(578)	(2,700)
Outstanding Principal Balance, Change in fair value recorded in earnings	0	0	0	0
Valuation Adjustment, Change in fair value recorded in earnings	423	2,413	1,985	(3,346)
Fair Value, Change in fair value recorded in earnings	423	2,413	1,985	(3,346)
Outstanding Principal Balance, Ending	30,094	66,232	30,094	66,232
Valuation Adjustment, Ending	(2,865)	(7,133)	(2,865)	(7,133)
Fair Value, Ending	$ 27,229	$ 59,099	$ 27,229	$ 59,099