SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) risk	Dec. 31, 2016 USD ($)
Cash and Cash Equivalents [Line Items]
Number of significant types of economic risk | risk	2
Monthly payment threshold period to determine whether loan is past due (in days)	30 days
Principal or interest past due threshold period (in days)	120 days
Restricted cash	$ 150,252	$ 163,095
General maturity term of repurchase agreements and notes payable (in years)	1 year
Ginnie Mae Excess MSRs
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 3,300	2,200
MSRs
Cash and Cash Equivalents [Line Items]
Restricted cash	28,600	0
Secured Corporate Note
Cash and Cash Equivalents [Line Items]
Restricted cash	9,900	22,300
Servicer Advance Investments
Cash and Cash Equivalents [Line Items]
Restricted cash	62,400	82,100
Consumer Loan
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 46,100	$ 56,400