REAL ESTATE INVESTMENTS - Summary of Office Properties Sold (Additional Information) (Details) $ in Thousands		3 Months Ended		12 Months Ended
	Mar. 15, 2017 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2017 USD ($) property	Dec. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain on disposition of real estate				$ 356	$ 2,932	$ 32,017
Number of Properties | property				95
Building and improvements				$ 3,049,395	3,048,889
Proceeds from the sale of properties				41,795	$ 324,090	171,860
Subaru National Training Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Direct finance lease, interest income		$ 1,600
Philadelphia Marine Center (Marine Piers)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payment by cash on purchases	$ 12,000
Proceeds from the sale of properties	11,200
Balance payment on purchase price	$ 9,400
Prior Period Disposals
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain on disposition of real estate				700
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
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
Other | Impairment on Held for Use Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Building and improvements				9,200	$ 38,000
Other | Residual Capitalization Rates
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Measurement input for impairment on assets held for sale			0.0900		0.075
Other | Discount Rates
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Measurement input for impairment on assets held for sale			0.0925		0.095
Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain on disposition of real estate				$ 1,000
Land | 11700, 11710, 11720 & 11740 Beltsville Drive (Calverton)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase price reduction						1,700
Provision for impairment on assets held for sale						$ 1,700