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REAL ESTATE INVESTMENTS - Held for Use Impairment and Sale (Narrative) (Details) | 12 Months Ended
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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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