COMMERCIAL REAL ESTATE INVESTMENTS - Internal CRE Debt and Preferred Equity Ratings (Details) - Commercial Mortgage $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($)
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	100.00%	100.00%
Performing	$ 673,783	$ 616,047
Performing - Closely Monitored	324,536	181,573
Performing - Special Mention	157,511	168,148
Substandard	100,993	67,390
Doubtful	0	0
Loss	0	0
Total	$ 1,256,823	$ 1,033,158
Number of rated loans (loan) | loan	2
Mezzanine Loans
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	28.70%	38.20%
Performing	$ 160,173	$ 206,169
Performing - Closely Monitored	51,608	66,498
Performing - Special Mention	111,711	122,348
Substandard	36,603	0
Doubtful	0	0
Loss	0	0
Total	$ 360,095	$ 395,015
Senior Mortgages
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	70.60%	60.90%
Performing	$ 513,610	$ 409,878
Performing - Closely Monitored	272,928	115,075
Performing - Special Mention	36,800	36,800
Substandard	64,390	67,390
Doubtful	0	0
Loss	0	0
Total	$ 887,728	$ 629,143
Preferred Equity Interests
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	0.70%	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
Total	$ 9,000	$ 9,000