MORTGAGE LOAN RECEIVABLES - Schedule of Mortgage Loans (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount		$ 3,423,668	$ 3,522,286
Provision for loan losses	[1]	(18,500)	(17,900)
Carrying Value		$ 3,387,157	$ 3,482,929
Weighted Average Yield		7.19%	7.76%
Remaining Maturity		1 year 9 months 21 days	1 year 9 months 7 days
First mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount		$ 3,116,050	$ 3,192,160
Carrying Value gross, consumer and commercial real estate		$ 3,098,241	$ 3,170,788
Weighted Average Yield		7.14%	7.70%
Remaining Maturity		1 year 3 months 7 days	1 year 2 months 4 days
Mezzanine loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount		$ 133,661	$ 148,221
Carrying Value gross, consumer and commercial real estate		$ 133,202	$ 147,602
Weighted Average Yield		10.87%	10.89%
Remaining Maturity		3 years 9 months 3 days	4 years 4 months 6 days
Mortgage loans held by consolidated subsidiaries
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount		$ 3,249,711	$ 3,340,381
Carrying Value gross, consumer and commercial real estate		$ 3,231,443	$ 3,318,390
Weighted Average Yield		7.29%	7.84%
Remaining Maturity		1 year 4 months 13 days	1 year 3 months 25 days
Total mortgage loan receivables held for investment, net, at amortized cost
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount		$ 3,249,711	$ 3,340,381
Provision for loan losses		(18,500)	(17,900)	$ (17,600)	$ (4,000)
Carrying Value		3,212,943	3,300,490
First mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount		173,957	181,905
Carrying Value		$ 174,214	$ 182,439
Weighted Average Yield		4.59%	5.46%
Remaining Maturity		9 years 8 months 4 days	9 years 9 months

[1] Includes amounts relating to consolidated variable interest entities. See Note 10.