Securitizations and Variable Interest Entities - Summary of Select Information Related to VIEs (Details) $ in Thousands		1 Months Ended	3 Months Ended	9 Months Ended
		May 31, 2018 USD ($) certificate	Sep. 30, 2018 USD ($) trust	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Restricted cash	[1]		$ 203,258	$ 203,258	$ 242,570
Securities available for sale at fair value			165,442	165,442	117,573
Loans held for investment at fair value	[1]		2,133,829	2,133,829	2,932,325
Loans held for investment by the Company at fair value	[1]		12,198	12,198	361,230
Loans held for sale by the Company at fair value (1)	[1]		459,283	459,283	235,825
Accrued interest receivable	[1]		24,546	24,546	33,822
Other assets	[1]		113,896	113,896	156,278
Total assets			3,589,968	3,589,968	4,640,831
Accrued interest payable	[1]		22,050	22,050	32,992
Accrued expenses and other liabilities	[1]		136,629	136,629	228,380
Notes, certificates and secured borrowings at fair value	[1]		2,152,316	2,152,316	2,954,768
Payable to securitization note and residual certificate holders	[1]		0	0	312,123
Credit facilities and securities sold under repurchase agreements	[1]		305,336	305,336	32,100
Liabilities			2,721,876	2,721,876	3,713,074
Consolidated VIEs
Variable Interest Entity [Line Items]
Restricted cash			31,855	31,855	34,370
Securities available for sale at fair value			0	0	0
Loans held for investment at fair value			770,901	770,901	1,202,260
Loans held for investment by the Company at fair value			0	0	350,699
Loans held for sale by the Company at fair value (1)			307,828	307,828	60,812
Accrued interest receivable			9,596	9,596	15,602
Other assets			2,456	2,456	6,324
Total assets			1,122,636	1,122,636	1,670,067
Accrued interest payable			8,804	8,804	14,789
Accrued expenses and other liabilities			1,214	1,214	52
Notes, certificates and secured borrowings at fair value			776,403	776,403	1,210,349
Payable to securitization note and residual certificate holders			0	0	312,123
Credit facilities and securities sold under repurchase agreements			185,752	185,752	32,100
Liabilities			972,173	972,173	1,569,413
Total net assets			150,463	150,463	100,654
Unconsolidated VIEs
Variable Interest Entity [Line Items]
Restricted cash			0	0	0
Securities available for sale at fair value			109,679	109,679	47,049
Loans held for investment at fair value			0	0	0
Loans held for investment by the Company at fair value					0
Loans held for sale by the Company at fair value (1)			0	0	0
Accrued interest receivable			1,053	1,053	407
Other assets			27,867	27,867	15,779
Total assets			138,599	138,599	63,235
Accrued interest payable			0	0	0
Accrued expenses and other liabilities			0	0	300
Notes, certificates and secured borrowings at fair value			0	0	0
Payable to securitization note and residual certificate holders					0
Credit facilities and securities sold under repurchase agreements			24,584	24,584	0
Liabilities			24,584	24,584	300
Total net assets			114,015	114,015	62,935
Total
Variable Interest Entity [Line Items]
Restricted cash			31,855	31,855	34,370
Securities available for sale at fair value			109,679	109,679	47,049
Loans held for investment at fair value			770,901	770,901	1,202,260
Loans held for investment by the Company at fair value					350,699
Loans held for sale by the Company at fair value (1)			307,828	307,828	60,812
Accrued interest receivable			10,649	10,649	16,009
Other assets			30,323	30,323	22,103
Total assets			1,261,235	1,261,235	1,733,302
Accrued interest payable			8,804	8,804	14,789
Accrued expenses and other liabilities			1,214	1,214	352
Notes, certificates and secured borrowings at fair value			776,403	776,403	1,210,349
Payable to securitization note and residual certificate holders					312,123
Credit facilities and securities sold under repurchase agreements			210,336	210,336	32,100
Liabilities			996,757	996,757	1,569,713
Total net assets			$ 264,478	264,478	$ 163,589
Gain on Sales of Loans
Variable Interest Entity [Line Items]
Deconsolidation gain				1,800
CLUB Certificates
Variable Interest Entity [Line Items]
Number of Series Trusts consolidated | certificate		2
Loss on consolidation		$ (500)
Number of Series Trusts dissolved | trust			2
Other investment fund
Variable Interest Entity [Line Items]
Other assets			$ 8,500	$ 8,500

[1] Includes amounts in consolidated variable interest entities (VIEs) presented separately in the table below.