LOANS - Internal Loan and Preferred Equity Ratings (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($)
Real Estate [Line Items]
Outstanding Principal	$ 96,782,059	$ 95,582,210
Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 1,443,203	$ 1,033,158
Percentage of CRE Debt and Preferred Equity Portfolio	100.00%	100.00%
Performing	$ 859,445	$ 616,047
Performing - Closely Monitored	366,671	181,573
Performing - Special Mention	116,094	168,148
Substandard	100,993	67,390
Doubtful	0	0
Loss	$ 0	0
Number of rated loans (loan) | loan	2
Commercial Mortgage | Mezzanine Loans
Real Estate [Line Items]
Outstanding Principal	$ 343,354	$ 395,015
Percentage of CRE Debt and Preferred Equity Portfolio	23.80%	38.20%
Performing	$ 135,334	$ 206,169
Performing - Closely Monitored	64,323	66,498
Performing - Special Mention	107,094	122,348
Substandard	36,603	0
Doubtful	0	0
Loss	0	0
Commercial Mortgage | Senior Mortgages
Real Estate [Line Items]
Outstanding Principal	$ 1,090,849	$ 629,143
Percentage of CRE Debt and Preferred Equity Portfolio	75.60%	60.90%
Performing	$ 724,111	$ 409,878
Performing - Closely Monitored	302,348	115,075
Performing - Special Mention	0	36,800
Substandard	64,390	67,390
Doubtful	0	0
Loss	0	0
Commercial Mortgage | Preferred Equity
Real Estate [Line Items]
Outstanding Principal	$ 9,000	$ 9,000
Percentage of CRE Debt and Preferred Equity Portfolio	0.60%	0.90%
Performing	$ 0	$ 0
Performing - Closely Monitored	0	0
Performing - Special Mention	9,000	9,000
Substandard	0	0
Doubtful	0	0
Loss	$ 0	$ 0