REAL ESTATE INVESTMENTS - Held for Sale and Use Impairment (Narrative) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) property	Dec. 31, 2019 USD ($) a ft² property	Dec. 31, 2018 USD ($) a property
Number of properties | property		95
Building and improvements		$ 3,049,395	$ 3,048,889
Assets held for sale, net		7,349	11,599
Land Held For Development | Assets Held-for-sale
Assets held for sale, net		$ 7,300	$ 11,600
Acres | a		35.2	37.9
Other
Number of properties | property	4		1
Impairment of properties held for use	$ 1,000		$ 14,800
Carrying value of real estate prior to impairment		$ 10,200	$ 52,800
Impairment hold period	10 years		10 years
Impairment on Held for Use Properties | Other
Building and improvements		$ 9,200	$ 38,000
Residual Capitalization Rates | Other
Measurement input for impairment on assets held for sale	0.0900		0.075
Discount Rates | Other
Measurement input for impairment on assets held for sale	0.0925		0.095
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
Office Properties | Metropolitan Washington, D.C. - Office | Assets Held-for-sale
Impairment hold period		10 years
Number of properties sold | property		8
Rentable square feet | ft²		1,293,197
Provision for impairment on assets held for sale		$ 56,900