REAL ESTATE INVESTMENTS - Summary of Office Properties Sold (Details) $ in Thousands												3 Months Ended		12 Months Ended
	Dec. 21, 2020 USD ($) ft² property	Jul. 21, 2020 USD ($) ft² property	Jun. 15, 2020 USD ($) a	Mar. 19, 2020 USD ($) ft²	Sep. 11, 2019 USD ($) ft²	Mar. 15, 2019 USD ($) a	Dec. 21, 2018 USD ($) ft²	Dec. 20, 2018 USD ($) ft²	Jun. 21, 2018 USD ($) ft²	Mar. 16, 2018 USD ($) a	Jan. 10, 2018 USD ($) a	Sep. 30, 2020 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale														$ 289,461	$ 356	$ 2,932
Sale of ownership interest percentage	60.00%
Mid-Atlantic Office Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Variable consideration	$ 4,100
Mid-Atlantic Office Portfolio | Minimum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Variable consideration holdback term	6 months
Mid-Atlantic Office Portfolio | Maximum
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Variable consideration holdback term	9 months
Commerce Square
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales price		$ 115,000
Sale of ownership interest percentage		30.00%
Value of assets disposed of		$ 600,000
Deconsolidation gain												$ 271,900
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
Office Properties | Mid-Atlantic Office JV
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	1,128,645
Sales price	$ 192,900
Office Properties | Mid-Atlantic Office Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	1,128,645
Sales price	$ 192,943
Gain (loss) on sale	$ 15,164
Number of properties sold | property	12
Office Properties | Mid-Atlantic Office Portfolio | Pennsylvania
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	9
Office Properties | Mid-Atlantic Office Portfolio | Maryland
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	3
Office Properties | Commerce Square
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²		1,896,142
Sales price		$ 115,000
Gain (loss) on sale		$ 271,905
Number of properties sold | property		2
Office Properties | 52 Swedesford Square
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²				131,077
Sales price				$ 18,000
Gain (loss) on sale				$ 2,336
Office Properties | 1900 Gallows Rd
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²					210,632
Sales price					$ 36,400
Gain (loss) on sale					$ (367)
Office Properties | Rockpoint Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²								1,293,197
Sales price								$ 312,000
Gain (loss) on sale								$ 397
Office Properties | 20 East Clementon Road
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²									38,260
Sales price									$ 2,000
Gain (loss) on sale									$ (35)
Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale															$ 1,000
Land | Keith Valley
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acres | a			14.0
Sales price			$ 4,000
Gain (loss) on sale			$ 201
Land | 9 Presidential Boulevard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acres | a						2.7
Sales price						$ 5,325
Gain (loss) on sale						$ 751
Land | Garza Ranch Office
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acres | a										6.6
Sales price										$ 14,571
Gain (loss) on sale										$ 1,515
Land | Westpark Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Acres | a											13.1
Sales price											$ 485
Gain (loss) on sale											$ 22
Land | Garza Ranch - Multi-family
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale																$ 3,000
Mixed-use properties | Subaru National Training Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²							83,000
Sales price							$ 45,300
Gain (loss) on sale							$ 2,570