LOANS - Internal Loan and Preferred Equity Ratings (Details) $ in Thousands	Sep. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Real Estate [Line Items]
Outstanding Principal	$ 114,328,368	$ 96,602,237
Balance of the loan loss provision	12,700	3,500
Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 1,441,437	$ 1,307,911
Percentage of CRE Debt and Preferred Equity Portfolio	100.00%	100.00%
Performing	$ 524,761	$ 793,842
Performing - Closely Monitored	572,963	254,676
Performing - Special Mention	189,910	151,400
Substandard	110,200	100,993
Doubtful	36,603	7,000
Loss (3)	$ 7,000	$ 0
Number of rated loans | loan	3	2
Balance of the loan loss provision	$ 5,700	$ 3,500
Commercial Mortgage | Mezzanine loans
Real Estate [Line Items]
Outstanding Principal	$ 192,065	$ 319,663
Percentage of CRE Debt and Preferred Equity Portfolio	13.30%	24.40%
Performing	$ 61,063	$ 140,776
Performing - Closely Monitored	70,299	38,884
Performing - Special Mention	0	96,400
Substandard	17,100	36,603
Doubtful	36,603	7,000
Loss (3)	7,000	0
Commercial Mortgage | Senior mortgages
Real Estate [Line Items]
Outstanding Principal	$ 434,949	$ 988,248
Percentage of CRE Debt and Preferred Equity Portfolio	30.20%	75.60%
Performing	$ 141,505	$ 653,066
Performing - Closely Monitored	185,954	215,792
Performing - Special Mention	64,390	55,000
Substandard	43,100	64,390
Doubtful	0	0
Loss (3)	0	0
Commercial Mortgage | Senior Securitized Mortgages
Real Estate [Line Items]
Outstanding Principal	$ 814,423	$ 0
Percentage of CRE Debt and Preferred Equity Portfolio	56.50%
Performing	$ 322,193
Performing - Closely Monitored	316,710
Performing - Special Mention	125,520
Substandard	50,000
Doubtful	0
Loss (3)	$ 0