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REAL ESTATE INVESTMENTS - Acquisitions (Textual) (Details) $ in Thousands |  |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended | 17 Months Ended
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Dec. 19, 2018 USD ($) ft² | Dec. 11, 2018 USD ($) ft² property | Jun. 29, 2018 USD ($) ft² | Mar. 22, 2018 USD ($) | Jan. 05, 2018 USD ($) ft² | Oct. 13, 2017 USD ($) ft² | Jul. 28, 2017 USD ($) ft² | Dec. 31, 2019 USD ($) ft² property | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2019 USD ($) ft² property | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Internal direct construction costs capitalized related to development of properties and land holdings |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,431 | $ 6,964 | $ 6,063 |
Number of properties | property |  |  |  |  |  |  |  | 95 |  |  |  |  |  |  |  |  | 95 |  |  |
Area of real estate property | ft² |  |  |  |  |  |  |  | 16,711,387 |  |  |  |  |  |  |  |  | 16,711,387 |  |  |
Gain on promoted interest in unconsolidated real estate venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | 28,283 | 0 |
Net gain from remeasurement of investment in real estate ventures |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,639 | 142,233 | 80,526 |
Prepaid leasehold interests in land held for development, net |  |  |  |  |  |  |  | $ 39,592 |  |  |  | $ 39,999 |  |  |  | $ 0 | 39,592 | 39,999 | 0 | $ 39,999
Capitalized transaction costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 15,485 | 18,407 | 17,657 |
Net income (loss) |  |  |  |  |  |  |  | $ 16,676 | $ 6,679 | $ 6,112 | $ 4,404 | $ 120,792 | $ (43,260) | $ 12,661 | $ 43,956 |  | $ 34,267 | 134,518 | 120,173 |
Office Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties | property |  |  |  |  |  |  |  | 86 |  |  |  |  |  |  |  |  | 86 |  |  |
Area of real estate property | ft² |  |  |  |  |  |  |  | 15,450,417 |  |  |  |  |  |  |  |  | 15,450,417 |  |  |
Seven Tower Bridge |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of ownership interest percentage |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Working capital obtained | 368 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate cash payment | $ 39,037 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service and other 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Working capital obtained |  | 24,865 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on promoted interest in unconsolidated real estate venture |  | 28,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net gain from remeasurement of investment in real estate ventures |  | 103,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate cash payment |  | $ 117,348 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interests |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,400 |  |  |
Net income (loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,300 |  |  |
3025 JFK Boulevard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lease agreement term |  |  | 99 years |  |  |  |  |  |  |  |  |  |  |  |  |  | 99 years |  |  |
Prepaid leasehold interests in land held for development, net |  |  | $ 15,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capitalized transaction costs |  |  | 300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
3025 JFK Boulevard | Other Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage debt assumed repaid at settlement |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Working capital obtained |  |  |  |  | (1,379) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate cash payment |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership interest after non-monetary exchange transaction |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of unencumbered acquisition related costs, to tangible and intangible assets and liabilities |  |  |  |  | $ 23,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,800 |  |  |
Net income (loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 300 |  |  |
Four Tower Bridge | Conshohocken | Pennsylvania |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of real estate property | ft² |  |  |  |  | 86,021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real estate property debt |  |  |  |  | $ 9,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Bulletin Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rentable square feet | ft² |  |  |  |  |  | 282,709 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross purchase value of office property |  |  |  |  |  | $ 35,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lease agreement term |  |  |  |  |  | 99 years |  |  |  |  |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,200 |  |  |  |
Net income (loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 400 |  |  |  |
3000 Market Street |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gross purchase value of office property |  |  |  |  |  |  | $ 32,700 |  |  |  |  |  |  |  |  |  |  |  |  |
Service and other revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 800
Net income (loss) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500
3000 Market Street | Office Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of real estate property | ft² |  |  |  |  |  |  | 58,587 |  |  |  |  |  |  |  |  |  |  |  |  |
Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capitalized interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,200 | $ 3,600 | $ 3,100 |

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