REAL ESTATE INVESTMENTS - Acquisitions Narrative (Details) $ in Thousands						12 Months Ended
	Dec. 19, 2018 USD ($) ft²	Dec. 11, 2018 USD ($) ft² property	Jun. 29, 2018 USD ($) ft²	Mar. 22, 2018 USD ($) ft²	Jan. 05, 2018 USD ($) ft²	Dec. 31, 2020 USD ($) ft² property	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 21, 2020	Nov. 05, 2020 USD ($)
Business Acquisition [Line Items]
Number of properties | property						82
Area of real estate property | ft²						13,852,464
Net gain on real estate venture transactions						$ 75	$ 11,639	$ 142,233
Gain on promoted interest in unconsolidated real estate venture						0	0	28,283
Prepaid leasehold interests in land held for development, net						39,185	39,592
Sale of ownership interest percentage									60.00%
Revenue						534,852	580,417	544,345
Net income (loss)						$ 305,527	$ 34,267	134,518
Seven Tower Bridge
Business Acquisition [Line Items]
Sale of ownership interest percentage					20.00%
1505-11 Race Street
Business Acquisition [Line Items]
Transaction costs capitalized										$ 300
Quarry Lake II
Business Acquisition [Line Items]
Rentable square feet | ft²	120,559
Gross purchase value of office property	$ 39,500
Mortgage debt assumed repaid at settlement	0
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Net Working Capital Assumed	368
Aggregate cash payment	$ 39,037
Revenue								100
Net income (loss)								100
DRA Austin Venture
Business Acquisition [Line Items]
Gross purchase value of office property		$ 535,100
Ownership percentage		50.00%
Number of properties | property		12
Area of real estate property | ft²		1,570,123
Net gain on real estate venture transactions		$ 103,800
Gain on promoted interest in unconsolidated real estate venture		28,300
Investment in real estate venture		14,600
Mortgage debt assumed repaid at settlement		115,461
Credit settlement for notes receivable provided to venture		130,700
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Net Working Capital Assumed		24,865
Aggregate cash payment		$ 117,348
Percentage of ownership interests		50.00%
Revenue								3,400
Net income (loss)								1,300
3025 JFK Boulevard
Business Acquisition [Line Items]
Lease agreement term			99 years
Acres | ft²			1
Prepaid leasehold interests in land held for development, net			$ 15,000
Capitalized transaction costs			300
Leasehold valuation credit			5,600
3001-3003 and 3025 JFK Boulevard
Business Acquisition [Line Items]
Leasehold valuation credit applied to development			$ 2,400
Minimum floor area ratio required for realization of credit (sq. feet) | ft²			1,200,000
Remaining leasehold valuation credit			$ 3,200
Reimbursable leasehold valuation credit			$ 3,200
3001-3003 JFK Boulevard
Business Acquisition [Line Items]
Lease agreement term				99 years
Acres | ft²				1
Prepaid leasehold interests in land held for development, net				$ 24,600
Capitalized transaction costs				$ 300
Four Tower Bridge
Business Acquisition [Line Items]
Ownership percentage					35.00%
Area of real estate property | ft²					86,021
Mortgage debt assumed repaid at settlement					$ 0
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Net Working Capital Assumed					(1,379)
Aggregate cash payment					$ 0
Percentage of ownership interest after non-monetary exchange transaction					100.00%
Real estate property debt					$ 9,700
Fair value of unencumbered acquisition related costs, to tangible and intangible assets and liabilities					$ 23,600
Revenue								2,800
Net income (loss)								$ 300