DISPOSITIONS AND IMPAIRMENTS - Real Estate Assets Held For Sale (Details) $ in Thousands	Sep. 30, 2021 USD ($) property	Dec. 31, 2020 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties Held for Sale | property	1,300
Assets held for sale	$ 316,769	$ 9,608
Liabilities related to assets held for sale	20,518	3,246
Triple-Net Leased Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	1,414	4,960
Liabilities related to assets held for sale	621	2,690
Office Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	12,813	15
Liabilities related to assets held for sale	2,096	101
Senior Living Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	302,542	4,633
Liabilities related to assets held for sale	$ 17,801	$ 455
Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties Held for Sale | property	32	2
Disposal Group, Held-for-sale, Not Discontinued Operations | Triple-Net Leased Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties Held for Sale | property	1	1
Disposal Group, Held-for-sale, Not Discontinued Operations | Office Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties Held for Sale | property	6	0
Disposal Group, Held-for-sale, Not Discontinued Operations | Senior Living Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties Held for Sale | property	25	1