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MORTGAGE LOAN RECEIVABLES - Additional Information (Details)				3 Months Ended								12 Months Ended
	Jan. 01, 2020 USD ($) loans $ / shares	Sep. 19, 2018	Mar. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) loan	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) loans	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Unamortized discounts included in mortgage loan receivables held for investment, at amortized cost				$ 400,000	$ 400,000	$ 400,000	$ 400,000
General CECL Reserve				49,457,000	49,457,000	49,457,000	49,457,000	$ 20,500,000				$ 20,500,000
Individually impaired loans				81,316,000	81,316,000	81,316,000	81,316,000
Provision expense for current expected credit loss (implementation impact)					17,993,000				$ 300,000
Provision for loan losses					26,581,000				300,000
Loans nonaccrual status, amount				142,387,000	142,387,000	142,387,000	$ 142,387,000	98,725,000				98,725,000
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Direct capitalization rate			4.70%				4.60%
Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Direct capitalization rate			5.00%				4.90%
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]
Loans receivable with variable rates of interest				$ 2,800,000,000	$ 2,800,000,000	$ 2,800,000,000	$ 2,800,000,000	$ 2,500,000,000				$ 2,500,000,000
Loans receivable with variable rates of interest				80.00%	80.00%	80.00%	80.00%	77.20%				77.20%
Loans receivable with variable rates of interest, subject to interest rate floors				100.00%	100.00%	100.00%	100.00%	100.00%				100.00%
General CECL Reserve				$ 49,457,000	$ 49,457,000	$ 49,457,000	$ 49,457,000	$ 20,500,000	18,200,000	$ 17,900,000		$ 20,500,000	$ 17,900,000
Provision expense for current expected credit loss (implementation impact)					4,964,000
Provision for loan losses					(2,000,000)
Loans in default, carrying value				$ 61,500,000	61,500,000	61,500,000	$ 61,500,000			45,000,000.0			$ 45,000,000.0
Total mortgage loan receivables held for investment, net, at amortized cost \| Two Company Loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number or loans in default				2			2						2
Loans in default, carrying value				$ 26,900,000	26,900,000	26,900,000	$ 26,900,000	26,900,000		46,400,000		$ 26,900,000	$ 46,400,000
Total mortgage loan receivables held for investment, net, at amortized cost \| One Company Loan
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan reserve amount								500,000
Number or loans in default			1				1					1	1	1
Loans in default, carrying value			$ 5,900,000	4,100,000	4,100,000	4,100,000	$ 4,100,000			4,100,000			$ 4,100,000	$ 5,900,000
Total mortgage loan receivables held for investment, net, at amortized cost \| One Company Loan \| Series A
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans in default, carrying value										35,000,000.0			35,000,000.0
Total mortgage loan receivables held for investment, net, at amortized cost \| One Company Loan \| Series B
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans in default, carrying value								10,400,000		$ 10,000,000.0		$ 10,400,000	$ 10,000,000.0
Total mortgage loan receivables held for investment, net, at amortized cost \| One Company Loan \| Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percentage of equity kicker not subject to investment										19.00%
Total mortgage loan receivables held for investment, net, at amortized cost \| One Company Loan \| Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Percentage of equity kicker not subject to investment										25.00%
Total mortgage loan receivables held for investment, net, at amortized cost \| Asset Specific Reserve, Company Loan
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loan reserve amount			$ 2,700,000								$ 10,000,000.0
Loans in default, carrying value				46,400,000	46,400,000	46,400,000	46,400,000	61,500,000				61,500,000
Terminal capitalization rate		7.50%
Discount rate											8.75%
Total mortgage loan receivables held for investment, net, at amortized cost \| Two Of Company Loans 2
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans in default, carrying value				7,700,000	7,700,000	7,700,000	7,700,000
Mortgage loan receivables held for sale
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans receivable with fixed rates of interest				$ 154,800,000	$ 154,800,000	$ 154,800,000	$ 154,800,000	$ 122,700,000				$ 122,700,000
Percentage of loans receivable with fixed rates of interest				100.00%	100.00%	100.00%	100.00%	100.00%				100.00%
Provision for loan losses									$ 0
Loan on non-accrual status
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans nonaccrual status, amount				$ 0	$ 0	$ 0	$ 0
Accounting Standards Update 2016-13
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
General CECL Reserve	$ 11,600,000			30,200,000	30,200,000	30,200,000	30,200,000
Increase of reserve to provision expenses					18,000,000.0
Percentage of total loan portfolio	0.36%
Carrying value of held for investment loan portfolio	$ 3,200,000,000
Provision expense for current expected credit loss (implementation impact)	$ 5,800,000
Book value of common stock (in usd per share) \| $ / shares	$ 0.05
Additional CECL reserve recorded				$ 18,600,000	18,600,000	$ 18,600,000	18,600,000
Loans that previously had asset-specific reserves	3			4		5
Increase in reserve of unfunded commitments				$ 600,000	600,000	$ 600,000	600,000
Accounting Standards Update 2016-13 \| Asset Specific Reserve, Company Loan
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
General CECL Reserve	$ 14,700,000			$ 20,700,000	$ 20,700,000	$ 20,700,000	$ 20,700,000
Carrying value of financing receivable	39,800,000
Aggregate amount of loan specific reserves	$ 7,500,000