Fair Value of Assets and Liabilities - Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 3,680,629	$ 4,629,567
Payable to securitization note holders	1,479	0
Total liabilities	2,965,512	4,328,654
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	107,544	287,137
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	3,573,085	4,342,430
Payable to securitization note holders	1,479
Total liabilities	2,965,512	4,328,654
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents (1)	401,719	515,602
Restricted cash	242,570	177,810
Servicer reserve receivable	13,685	4,938
Deposits	855	855
Total assets	658,829	699,205
Accrued expenses and other liabilities	13,856	10,981
Accounts payable	11,151	10,889
Payable to investors	143,310	125,884
Payable to securitization note holders	310,644
Warehouse notes payable	32,100
Total liabilities	511,061	147,754
Portion at Other than Fair Value Measurement | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents (1)	0	0
Restricted cash	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
Payable to investors	0	0
Payable to securitization note holders	0
Warehouse notes payable	0
Total liabilities	0	0
Portion at Other than Fair Value Measurement | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents (1)	401,719	515,602
Restricted cash	242,570	177,810
Servicer reserve receivable	13,685	4,938
Deposits	855	855
Total assets	658,829	699,205
Accrued expenses and other liabilities	0	0
Accounts payable	11,151	10,889
Payable to investors	143,310	125,884
Payable to securitization note holders	310,644
Warehouse notes payable	0
Total liabilities	465,105	136,773
Portion at Other than Fair Value Measurement | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents (1)	0	0
Restricted cash	0	0
Servicer reserve receivable	0	0
Deposits	0	0
Total assets	0	0
Accrued expenses and other liabilities	13,856	10,981
Accounts payable	0	0
Payable to investors	0	0
Payable to securitization note holders	0
Warehouse notes payable	32,100
Total liabilities	45,956	10,981
Balance at Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents (1)	401,719	515,602
Restricted cash	242,570	177,810
Servicer reserve receivable	13,685	4,938
Deposits	855	855
Total assets	658,829	699,205
Accrued expenses and other liabilities	13,856	10,981
Accounts payable	11,151	10,889
Payable to investors	143,310	125,884
Payable to securitization note holders	310,644
Warehouse notes payable	32,100
Total liabilities	$ 511,061	$ 147,754