DESCRIPTION OF BUSINESS - COVID-19 Update (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	May 06, 2020	Mar. 31, 2020	Mar. 12, 2020
Unusual or Infrequent Item, or Both [Line Items]
Dividends declared (in dollars per share)	$ 0.45		$ 0.45		$ 0.45			$ 0.7925
Impairment of real estate					$ 121,500,000	$ 13,500,000
Allowance on loans receivable and investments			$ 29,655,000		29,655,000
Non-Mortgage Loans Receivable, Net
Unusual or Infrequent Item, or Both [Line Items]
Allowance on loans receivable and investments					24,500,000
COVID-19 Impact
Unusual or Infrequent Item, or Both [Line Items]
Decrease in dividend payable			43.00%
Cash, quarterly dividend reserve	$ 130,000,000		$ 130,000,000		130,000,000
Reduction in expected future payments for capital improvements							$ 300,000,000
Expected future payments for capital improvements	$ 500,000,000		500,000,000		500,000,000
Percent of corporate positions eliminated	25.00%
Impairment of real estate			$ 108,800,000
Impairment of real estate, percent of total			1.00%
Impairment on equity investment					10,700,000
Deferred tax assets, valuation allowance	$ 56,400,000		$ 56,400,000		56,400,000
COVID-19 Impact | Forecast | Minimum
Unusual or Infrequent Item, or Both [Line Items]
Decrease in general and administrative		$ 25,000,000
COVID-19 Impact | Forecast | Maximum
Unusual or Infrequent Item, or Both [Line Items]
Decrease in general and administrative		$ 30,000,000
COVID-19 Impact | CEO | Forecast
Unusual or Infrequent Item, or Both [Line Items]
Reduction in salary				20.00%
COVID-19 Impact | Executive Officers | Forecast
Unusual or Infrequent Item, or Both [Line Items]
Reduction in salary				10.00%
COVID-19 Impact | Fair Value
Unusual or Infrequent Item, or Both [Line Items]
Real estate investments	192,800,000		192,800,000		192,800,000
COVID-19 Impact | Non-Mortgage Loans Receivable, Net
Unusual or Infrequent Item, or Both [Line Items]
Allowance on loans receivable and investments					20,800,000
COVID-19 Impact | Government-sponsored pooled loan investments, net
Unusual or Infrequent Item, or Both [Line Items]
Allowance on loans receivable and investments					8,800,000
Revolving Credit Facility | Unsecured revolving credit facility (1) | COVID-19 Impact
Unusual or Infrequent Item, or Both [Line Items]
Additional borrowing, notice to lender									$ 2,750,000,000
Maximum borrowing capacity	3,000,000,000.0		$ 3,000,000,000.0		3,000,000,000.0
Office Operations | COVID-19 Impact
Unusual or Infrequent Item, or Both [Line Items]
Lease, percent collected			99.00%
Triple-Net Leased Properties | COVID-19 Impact
Unusual or Infrequent Item, or Both [Line Items]
Loans and leases receivable, allowance	$ 54,200,000		$ 54,200,000		$ 54,200,000