MORTGAGE LOAN RECEIVABLES - Schedule of Mortgage Loans (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 3,896,886	$ 3,533,236
Provision for loan losses	(7,300)	(4,000)
Carrying Value	$ 3,864,616	$ 3,508,642
Weighted Average Yield (as a percent)	7.65%	7.03%
Remaining Maturity (years)	1 year 8 months 15 days	2 years 14 days
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	$ 3,788,546	$ 3,300,709
Carrying Value	3,764,172	3,282,462
Provision for loan losses	(7,300)	(4,000)	$ (4,000)	$ (4,000)
Carrying Value	$ 3,756,872	$ 3,278,462
Weighted Average Yield (as a percent)	7.70%	7.18%
Remaining Maturity (years)	1 year 5 months 23 days	1 year 7 months 9 days
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	$ 108,340	$ 232,527
Carrying Value	$ 107,744	$ 230,180
Weighted Average Yield (as a percent)	5.24%	4.88%
Remaining Maturity (years)	9 years 9 months 14 days	8 years 2 months 1 day