MORTGAGE LOAN RECEIVABLES - Schedule of Mortgage Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 3,533,236		$ 2,371,827
Provision for loan losses	(4,000)	[1]	(4,000)
Carrying Value	$ 3,508,642		$ 2,353,977
Weighted Average Yield (as a percent)	7.03%		6.73%
Remaining Maturity (years)	2 years 14 days		2 years 1 month 6 days
Mortgage loans held by consolidated subsidiaries
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 3,300,709		$ 2,011,309
Carrying Value	$ 3,282,462		$ 2,000,095
Weighted Average Yield (as a percent)	7.18%		7.17%
Remaining Maturity (years)	1 year 7 months 9 days		1 year 7 months 27 days
Mortgage loan receivables held for investment, net, at amortized cost
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	$ 3,300,709		$ 2,011,309
Provision for loan losses	(4,000)		(4,000)	$ (3,700)	$ (3,100)
Carrying Value	3,278,462		1,996,095
Mortgage loan receivables held for sale
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	232,527		360,518
Carrying Value	$ 230,180		$ 357,882
Weighted Average Yield (as a percent)	4.88%		4.20%
Remaining Maturity (years)	8 years 2 months 1 day		4 years 6 months 18 days

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.