COMMERCIAL REAL ESTATE INVESTMENTS - Internal CRE Debt and Preferred Equity Ratings (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)
Real Estate [Line Items]
Number of transferred investments (loan) | loan	1
Commercial Mortgage
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	100.00%	100.00%
Performing	$ 616,047	$ 398,771
Performing - Closely Monitored	181,573	413,487
Performing - Special Mention	168,148	162,757
Substandard	67,390	0
Doubtful	0	0
Loss	0	0
Total	$ 1,033,158	$ 975,015
Commercial Mortgage | Mezzanine loans
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	38.20%	46.50%
Performing	$ 206,169	$ 254,337
Performing - Closely Monitored	66,498	170,039
Performing - Special Mention	122,348	29,317
Substandard	0	0
Doubtful	0	0
Loss	0	0
Total	$ 395,015	$ 453,693
Commercial Mortgage | Senior mortgages
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	60.90%	52.60%
Performing	$ 409,878	$ 144,434
Performing - Closely Monitored	115,075	243,448
Performing - Special Mention	36,800	124,440
Substandard	67,390	0
Doubtful	0	0
Loss	0	0
Total	$ 629,143	$ 512,322
Commercial Mortgage | Preferred equity
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	0.90%	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