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INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Brandywine - AI Venture (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) property | Dec. 31, 2019 USD ($) ft² | Dec. 31, 2018 USD ($) ft² property
Schedule of Equity Method Investments [Line Items] |  |  |
Impairment of properties held for use | $ 0 | $ 0 |
Outstanding indebtedness | 1,830,595,000 | 2,144,418,000 |
Net gain on real estate venture transactions | 75,000 | 11,639,000 | $ 142,233,000
Proceeds from real estate venture sales | $ 0 | 9,730,000 | 60,346,000
Fairview Properties |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Other-than-temporary impairment |  |  | $ 4,100,000
Fairview Properties | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Number of office properties owned | property | 1 |  | 2
Impairment of properties held for use |  | $ 5,600,000 | $ 20,800,000
Impairment hold period |  |  | 10 years
Falls Church, Virginia | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Rentable square feet | ft² |  | 180,659 |
Outstanding indebtedness |  | $ 26,000,000.0 |
Net gain on real estate venture transactions |  | 2,200,000 |
Station Square | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Rentable square feet | ft² |  |  | 510,202
Net gain on real estate venture transactions |  |  | $ 1,500,000
Number of properties sold | property |  |  | 3
Gross sales price |  |  | $ 107,000,000.0
Encumbrances |  |  | 66,500,000
Debt prepayment penalty |  |  | 700,000
Gross proceeds from sale |  |  | 34,800,000
Proceeds from real estate venture sales |  |  | $ 17,400,000
Residual Capitalization Rates | Fairview Properties | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Measurement input |  |  | 0.080
Minimum | Discount Rates | Fairview Properties | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Measurement input |  |  | 0.090
Maximum | Discount Rates | Fairview Properties | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Measurement input |  |  | 0.095
Brandywine Realty Trust | Fairview Properties | Brandywine - AI Venture LLC |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Impairment of properties held for use |  | $ 2,800,000 | $ 10,400,000

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