Fair Value of Assets and Liabilities - Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 2,830,912	$ 3,680,629
Payable to securitization note holders	0	1,479
Total liabilities	2,162,328	2,965,512
Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Payable to securitization note holders		0
Total liabilities	0	0
Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	122,818	107,544
Payable to securitization note holders		0
Total liabilities	0	0
Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	2,708,094	3,573,085
Payable to securitization note holders		1,479
Total liabilities	2,162,328	2,965,512
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	348,018	401,719
Restricted cash (1)	203,258	242,570
Servicer reserve receivable	968	13,685
Deposits	860	855
Total assets	553,104	658,829
Accrued expenses and other liabilities	17,226	13,856
Accounts payable	8,778	11,151
Payables to investors	96,767	143,310
Payable to securitization note holders		310,644
Credit facilities and securities sold under repurchase agreements	305,336
Payable to revolving credit facilities		32,100
Total liabilities	428,107	511,061
Portion Not Recorded at Fair Value | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Restricted cash (1)	0	0
Servicer reserve receivable	0	0
Deposits	0	0
Total assets	0	0
Accrued expenses and other liabilities	0	0
Accounts payable	0	0
Payables to investors	0	0
Payable to securitization note holders		0
Credit facilities and securities sold under repurchase agreements	0
Payable to revolving credit facilities		0
Total liabilities	0	0
Portion Not Recorded at Fair Value | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	348,018	401,719
Restricted cash (1)	203,258	242,570
Servicer reserve receivable	968	13,685
Deposits	860	855
Total assets	553,104	658,829
Accrued expenses and other liabilities	0	0
Accounts payable	8,778	11,151
Payables to investors	96,767	143,310
Payable to securitization note holders		310,644
Credit facilities and securities sold under repurchase agreements	24,584
Payable to revolving credit facilities		0
Total liabilities	130,129	465,105
Portion Not Recorded at Fair Value | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Restricted cash (1)	0	0
Servicer reserve receivable	0	0
Deposits	0	0
Total assets	0	0
Accrued expenses and other liabilities	17,226	13,856
Accounts payable	0	0
Payables to investors	0	0
Payable to securitization note holders		0
Credit facilities and securities sold under repurchase agreements	280,752
Payable to revolving credit facilities		32,100
Total liabilities	297,978	45,956
Balance at Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	348,018	401,719
Restricted cash (1)	203,258	242,570
Servicer reserve receivable	968	13,685
Deposits	860	855
Total assets	553,104	658,829
Accrued expenses and other liabilities	17,226	13,856
Accounts payable	8,778	11,151
Payables to investors	96,767	143,310
Payable to securitization note holders		310,644
Credit facilities and securities sold under repurchase agreements	305,336
Payable to revolving credit facilities		32,100
Total liabilities	$ 428,107	$ 511,061