COMMERCIAL REAL ESTATE INVESTMENTS - Internal CRE Debt and Preferred Equity Ratings (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Mar. 31, 2018 USD ($)
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	$ 656,732
Performing - Closely Monitored	181,573	197,773
Performing - Special Mention	168,148	163,538
Substandard	67,390	67,390
Doubtful	0	0
Loss	0	0
Total	$ 1,033,158	$ 1,085,433
Commercial Mortgage | Mezzanine Loans
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	38.20%	36.00%
Performing	$ 206,169	$ 216,740
Performing - Closely Monitored	66,498	56,498
Performing - Special Mention	122,348	117,738
Substandard	0	0
Doubtful	0	0
Loss	0	0
Total	$ 395,015	$ 390,976
Commercial Mortgage | Senior Mortgages
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	60.90%	63.20%
Performing	$ 409,878	$ 439,992
Performing - Closely Monitored	115,075	141,275
Performing - Special Mention	36,800	36,800
Substandard	67,390	67,390
Doubtful	0	0
Loss	0	0
Total	$ 629,143	$ 685,457
Commercial Mortgage | Preferred Equity Interests
Real Estate [Line Items]
Percentage of CRE Debt and Preferred Equity Portfolio	0.90%	0.80%
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