Securitizations and Variable Interest Entities - Summary of Select Information Related to VIEs (Details) $ in Thousands		1 Months Ended	3 Months Ended	6 Months Ended
		May 31, 2018 USD ($) trust	Jun. 30, 2018 USD ($) trust	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Cash and cash equivalents	[1]		$ 434,179	$ 434,179	$ 401,719
Restricted cash	[1]		221,688	221,688	242,570
Securities available for sale at fair value			149,804	149,804	117,573
Loans held for investment at fair value	[1]		2,358,629	2,358,629	2,932,325
Loans held for investment by the Company at fair value	[1]		9,621	9,621	361,230
Loans held for sale by the Company at fair value (1)	[1]		515,307	515,307	235,825
Accrued interest receivable	[1]		26,635	26,635	33,822
Other assets	[1]		102,396	102,396	156,278
Total assets			3,949,083	3,949,083	4,640,831
Accrued interest payable	[1]		23,609	23,609	32,992
Accrued expenses and other liabilities	[1]		200,098	200,098	228,380
Notes, certificates and secured borrowings at fair value	[1]		2,377,080	2,377,080	2,954,768
Payable to securitization note and residual certificate holders	[1]		0	0	312,123
Payable to credit facilities	[1]		349,232	349,232	32,100
Total liabilities			3,074,863	3,074,863	3,713,074
Consolidated VIEs
Variable Interest Entity [Line Items]
Cash and cash equivalents			29,479	29,479	0
Restricted cash			37,311	37,311	34,370
Securities available for sale at fair value			0	0	0
Loans held for investment at fair value			893,282	893,282	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)			389,491	389,491	60,812
Accrued interest receivable			11,941	11,941	15,602
Other assets			2,936	2,936	6,324
Total assets			1,364,440	1,364,440	1,670,067
Accrued interest payable			9,806	9,806	14,789
Accrued expenses and other liabilities			1,227	1,227	52
Notes, certificates and secured borrowings at fair value			899,250	899,250	1,210,349
Payable to securitization note and residual certificate holders			0	0	312,123
Payable to credit facilities			249,232	249,232	32,100
Total liabilities			1,159,515	1,159,515	1,569,413
Total net assets			204,925	204,925	100,654
Unconsolidated VIEs
Variable Interest Entity [Line Items]
Cash and cash equivalents			0	0
Restricted cash			0	0	0
Securities available for sale at fair value			93,714	93,714	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			939	939	407
Other assets			22,773	22,773	15,779
Total assets			117,426	117,426	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
Payable to credit facilities			0	0	0
Total liabilities			0	0	300
Total net assets			117,426	117,426	62,935
Total
Variable Interest Entity [Line Items]
Cash and cash equivalents			29,479	29,479
Restricted cash			37,311	37,311	34,370
Securities available for sale at fair value			93,714	93,714	47,049
Loans held for investment at fair value			893,282	893,282	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)			389,491	389,491	60,812
Accrued interest receivable			12,880	12,880	16,009
Other assets			25,709	25,709	22,103
Total assets			1,481,866	1,481,866	1,733,302
Accrued interest payable			9,806	9,806	14,789
Accrued expenses and other liabilities			1,227	1,227	352
Notes, certificates and secured borrowings at fair value			899,250	899,250	1,210,349
Payable to securitization note and residual certificate holders					312,123
Payable to credit facilities			249,232	249,232	32,100
Total liabilities			1,159,515	1,159,515	1,569,713
Total net assets			$ 322,351	322,351	$ 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 | trust		2
Ownership interest in investment (percent)		100.00%
Loss on consolidation		$ (500)
Number of Series Trusts dissolved | trust			2
Other investment fund
Variable Interest Entity [Line Items]
Ownership (percent)			24.00%	24.00%
Other assets			$ 8,500	$ 8,500

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