LOANS - Internal Loan and Preferred Equity Ratings (Details) $ in Thousands	Jun. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Real Estate [Line Items]
Outstanding Principal	$ 119,035,166	$ 96,602,237
Balance of the loan loss provision	9,200	3,500
Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 1,429,914	$ 1,307,911
Percentage of CRE Debt and Preferred Equity Portfolio	100.00%	100.00%
Performing	$ 740,350	$ 793,842
Performing - Closely Monitored	275,913	254,676
Performing - Special Mention	305,658	151,400
Substandard	64,390	100,993
Doubtful	43,603	7,000
Loss	$ 0	$ 0
Number of rated loans | loan	1	2
Senior mortgages | Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 351,907	$ 988,248
Percentage of CRE Debt and Preferred Equity Portfolio	24.60%	75.60%
Performing	$ 221,112	$ 653,066
Performing - Closely Monitored	22,766	215,792
Performing - Special Mention	43,639	55,000
Substandard	64,390	64,390
Doubtful	0	0
Loss	0	0
Senior Securitized Mortgages | Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 795,052	0
Percentage of CRE Debt and Preferred Equity Portfolio	55.60%
Performing	$ 439,395
Performing - Closely Monitored	203,464
Performing - Special Mention	152,193
Substandard	0
Doubtful	0
Loss	0
Mezzanine loans | Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 282,955	$ 319,663
Percentage of CRE Debt and Preferred Equity Portfolio	19.80%	24.40%
Performing	$ 79,843	$ 140,776
Performing - Closely Monitored	49,683	38,884
Performing - Special Mention	109,826	96,400
Substandard	0	36,603
Doubtful	43,603	7,000
Loss	$ 0	$ 0