ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Beneficiaries' Equity, beginning balance		$ 1,688,303	$ 1,811,028	$ 1,826,870
Unrealized gain (loss) on derivative financial instruments		(5,972)	(8,210)	1,478
Allocation of unrealized (gains)/losses on derivative financial instruments to noncontrolling interests		29	41	(39)
Amortization of interest rate contracts reclassified from comprehensive income to interest expense	[1]	752	770	1,191
Beneficiaries' Equity, ending balance		1,804,648	1,688,303	1,811,028
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Partners' Equity, beginning balance		1,672,915	1,798,508	1,799,955
Unrealized gain (loss) on derivative financial instruments		(5,972)	(8,210)	1,478
Amortization of interest rate contracts reclassified from comprehensive income to interest expense	[1]	752	770	1,191
Partners' Equity, ending balance		1,793,082	1,672,915	1,798,508
Accumulated Other Comprehensive Income (Loss)
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Beneficiaries' Equity, beginning balance		(2,370)	5,029	2,399
Beneficiaries' Equity, ending balance		(7,561)	(2,370)	5,029
Accumulated Other Comprehensive Income (Loss) | BRANDYWINE OPERATING PARTNERSHIP, L.P.
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Partners' Equity, beginning balance		(2,715)	4,725	2,056
Partners' Equity, ending balance		$ (7,935)	$ (2,715)	$ 4,725

[1] Amounts reclassified from comprehensive income to interest expense within the Consolidated Statements of Operations.