LOANS - Internal Loan and Preferred Equity Ratings (Details)	3 Months Ended			12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) loan
Real Estate [Line Items]
Outstanding Principal	$ 106,622,443,000	$ 96,602,237,000		$ 96,602,237,000
Loan loss provision recorded during period	5,703,000	3,500,000	$ 0
Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 1,474,288,000	$ 1,307,911,000		$ 1,307,911,000
Percentage of CRE Debt and Preferred Equity Portfolio	100.00%	100.00%		100.00%
Performing	$ 882,458,000	$ 793,842,000		$ 793,842,000
Performing - Closely Monitored	332,437,000	254,676,000		254,676,000
Performing - Special Mention	151,400,000	151,400,000		151,400,000
Substandard	64,390,000	100,993,000		100,993,000
Doubtful	43,603,000	7,000,000		7,000,000
Loss	0	0		$ 0
Number of rated loans | loan				1
Senior mortgages | Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 405,968,000	$ 988,248,000		$ 988,248,000
Percentage of CRE Debt and Preferred Equity Portfolio	27.60%	75.60%		75.60%
Performing	$ 276,479,000	$ 653,066,000		$ 653,066,000
Performing - Closely Monitored	65,099,000	215,792,000		215,792,000
Performing - Special Mention	0	55,000,000		55,000,000
Substandard	64,390,000	64,390,000		64,390,000
Doubtful	0	0		0
Loss	0	0		0
Senior Securitized Mortgages | Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 739,058,000	0		0
Percentage of CRE Debt and Preferred Equity Portfolio	50.10%
Performing	$ 466,258,000
Performing - Closely Monitored	217,800,000
Performing - Special Mention	55,000,000
Substandard	0
Doubtful	0
Loss	0
Mezzanine loans | Commercial Mortgage
Real Estate [Line Items]
Outstanding Principal	$ 329,262,000	$ 319,663,000		$ 319,663,000
Percentage of CRE Debt and Preferred Equity Portfolio	22.30%	24.40%		24.40%
Performing	$ 139,721,000	$ 140,776,000		$ 140,776,000
Performing - Closely Monitored	49,538,000	38,884,000		38,884,000
Performing - Special Mention	96,400,000	96,400,000		96,400,000
Substandard	0	36,603,000		36,603,000
Doubtful	43,603,000	7,000,000		7,000,000
Loss	$ 0	$ 0		$ 0