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REAL ESTATE INVESTMENTS - Held for Use Impairment and Sale (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) a property	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) ft² property
Impairment of properties held for use	$ 0	$ 0
Number of properties | property	82
Building and improvements	$ 2,665,232,000	3,049,395,000
Assets held for sale, net	$ 7,349,000	$ 7,349,000
Other
Impairment of properties held for use			$ 14,800,000
Number of properties | property			1
Carrying value of real estate prior to impairment			$ 52,800,000
Building and improvements			$ 38,000,000.0
Impairment hold period			10 years
Other | Land Held For Development | Assets Held-for-sale
Number of properties | property	2
Assets held for sale, net	$ 7,300,000
Acres | a	35.2
Residual Capitalization Rates | Other
Measurement input for impairment on assets held for sale			0.075
Discount Rates | Other
Measurement input for impairment on assets held for sale			0.095
Metropolitan Washington, D.C. - Office | Assets Held-for-sale
Number of properties | property			8
Impairment hold period			10 years
Rentable square feet | ft²			1,293,197
Provision for impairment on assets held for sale			$ 56,900,000
Metropolitan Washington, D.C. - Office | Residual Capitalization Rates | Assets Held-for-sale
Measurement input for impairment on assets held for sale			0.0747
Metropolitan Washington, D.C. - Office | Discount Rates | Assets Held-for-sale
Measurement input for impairment on assets held for sale			0.0860

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