LOANS HELD FOR INVESTMENT - Loans Held for Investment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 1,682,498,000	$ 1,524,873,000
Outstanding principal	$ 1,692,894,000	$ 1,534,743,000
Unleveraged Effective Yield	6.80%	7.10%
Weighted Average Remaining Life	1 year 7 months 6 days	1 year 9 months 18 days
Mortgage loans on real estate	$ 0
Senior mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount	1,622,666,000	$ 1,489,708,000
Outstanding principal	$ 1,632,164,000	$ 1,498,530,000
Unleveraged Effective Yield	6.50%	7.00%
Weighted Average Remaining Life	1 year 6 months	1 year 8 months 12 days
Subordinated debt and preferred equity investments
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 59,832,000	$ 35,165,000
Outstanding principal	$ 60,730,000	$ 36,213,000
Unleveraged Effective Yield	15.10%	14.90%
Weighted Average Remaining Life	2 years 7 months 6 days	4 years 3 months 18 days