ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Beneficiaries' Equity, Beginning Balance		$ 1,811,028	$ 1,826,870	$ 1,864,388
Partners' Equity, Beginning Balance		1,798,508
Unrealized gain (loss) on derivative financial instruments		(8,210)	1,478	2,948
Allocation of unrealized (gains)/losses on derivative financial instruments to noncontrolling interests		41	(39)	(34)
Amortization of interest rate contracts reclassified from comprehensive income to interest expense	[1]	770	1,191	1,230
Beneficiaries' Equity, Ending Balance		1,688,303	1,811,028	1,826,870
Partners' Equity, Ending Balance			1,798,508
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Partners' Equity, Beginning Balance		1,798,508	1,799,955	1,840,593
Unrealized gain (loss) on derivative financial instruments		(8,210)	1,478	2,948
Amortization of interest rate contracts reclassified from comprehensive income to interest expense	[1]	770	1,191	1,230
Partners' Equity, Ending Balance		1,672,915	1,798,508	1,799,955
Accumulated Other Comprehensive Income (Loss)
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Beneficiaries' Equity, Beginning Balance		5,029	2,399	(1,745)
Beneficiaries' Equity, Ending Balance		(2,370)	5,029	2,399
Accumulated Other Comprehensive Income (Loss) | BRANDYWINE OPERATING PARTNERSHIP, L.P.
Schedule of Accumulated other comprehensive income (loss) [Line Items]
Partners' Equity, Beginning Balance		4,725	2,056	(2,122)
Partners' Equity, Ending Balance		$ (2,715)	$ 4,725	$ 2,056

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