INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Brandywine - AI Venture (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($) ft²
Schedule of Equity Method Investments [Line Items]
Outstanding indebtedness	$ 2,144,418	$ 2,028,046
Net gain on real estate venture transactions	11,639	142,233	$ 80,526
Encumbrances	315,437
Proceeds from real estate venture sales	$ 9,730	60,346	145,416
Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Other-than-temporary impairment		$ 4,076	$ 4,844
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
Number of properties sold | property		3
Gross sales price		$ 107,000
Encumbrances		66,500
Debt prepayment penalty		700
Gross proceeds from sale		34,800
Proceeds from real estate venture sales		17,400
Fairview Properties
Schedule of Equity Method Investments [Line Items]
Other-than-temporary impairment		4,100
Fairview Properties | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Number of office properties owned | property	1
Impairment of properties held for use	$ 5,600	$ 20,800
Impairment hold period		10 years
7101 Washington Avenue | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Rentable square feet | ft²			230,904
Net gain on real estate venture transactions			$ 13,800
Encumbrances			37,400
Debt prepayment penalty			800
Gross proceeds from sale			63,600
Proceeds from real estate venture sales			105,700
Other-than-temporary impairment			$ 4,800
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
Net gain on real estate venture transactions	2,200
Residual Capitalization Rates | Fairview Properties | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Measurement input		0.080
Residual Capitalization Rates | 7101 Washington Avenue | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Measurement input			0.075
Minimum | Discount Rates | Fairview Properties | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Measurement input		0.090
Minimum | Discount Rates | 7101 Washington Avenue | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Measurement input			0.078
Maximum | Discount Rates | Fairview Properties | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Measurement input		0.095
Maximum | Discount Rates | 7101 Washington Avenue | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Measurement input			0.085
Brandywine Realty Trust | Fairview Properties | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Impairment of properties held for use	$ 2,800	$ 10,400
Brandywine Realty Trust | 7101 Washington Avenue | Brandywine - AI Venture LLC
Schedule of Equity Method Investments [Line Items]
Proceeds from real estate venture sales			$ 31,800