MORTGAGE LOAN RECEIVABLES - Mortgage Loan Receivables by Loan Type (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 3,248,433		$ 3,522,286
Mortgage loan receivables held-for-sale	3,213,334		3,482,929
Provision for loan losses	(18,500)	$ (18,200)	(17,900)	$ (7,300)	$ (7,000)	$ (4,000)
First mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	2,992,553		3,192,160
Carrying Value	2,976,625		3,170,788
Mezzanine loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	143,781		148,221
Carrying Value	143,232		147,602
Mortgage loan receivables held for investment, net, at amortized cost
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	3,136,334		3,340,381
Carrying Value	3,119,857		3,318,390
Mortgage loan receivables held-for-sale	3,101,357		3,300,490
First mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	112,099		181,905
Mortgage loan receivables held-for-sale	111,977		182,439
Mortgage loan receivables held for sale
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	112,099		181,905
Mortgage loan receivables held-for-sale	$ 111,977		$ 182,439