MORTGAGE LOAN RECEIVABLES - Schedule of Mortgage Loans (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 2,144,704	$ 1,954,878
Provision for loan losses	(3,550)	(3,100)	$ (3,400)	$ (2,950)	$ (2,800)	$ (2,500)
Carrying Value	2,128,343	1,939,008
Mortgage loan receivables held for investment, at amortized cost
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	1,811,216	1,536,923
Carrying Value, gross	1,798,362	1,524,153
Provision for loan losses	(3,550)	(3,100)
Carrying Value	$ 1,794,812	$ 1,521,053
Weighted Average Yield (as a percent)	7.61%	7.33%
Remaining Maturity (years)	1 year 6 months 29 days	1 year 11 months 15 days
Mortgage loan receivables held for sale
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 333,488	$ 417,955
Carrying Value	$ 333,531	$ 417,955
Weighted Average Yield (as a percent)	4.15%	4.31%
Remaining Maturity (years)	7 years 7 months 6 days	9 years 8 months 19 days