SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revised Statements of Equity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity	$ 1,688,303	$ 1,811,028	$ 1,826,870	$ 1,864,388
Total partners' equity		1,798,508
Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity	3,192,158	3,200,312	3,218,077	3,258,386
Cumulative Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity	804,556	775,625	641,093	520,914
Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity	10,426	12,201	17,258	16,933
As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		1,832,573
Total partners' equity		1,820,053
As previously reported | Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		3,200,850	3,218,564	3,258,870
As previously reported | Cumulative Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		796,513	660,174	539,319
As previously reported | Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		12,320	17,420	17,093
Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		(21,545)
Total partners' equity		(21,545)
Adjustments | Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		(538)	(487)	(484)
Adjustments | Cumulative Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		(20,888)	(19,081)	(18,405)
Adjustments | Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total beneficiaries' equity		(119)	(162)	(160)
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total partners' equity	1,672,915	1,798,508	1,799,955	1,840,593
BRANDYWINE OPERATING PARTNERSHIP, L.P. | Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total partners' equity	1,091	2,192	2,215	2,150
BRANDYWINE OPERATING PARTNERSHIP, L.P. | General Partner Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total partners' equity	$ 1,674,539	1,791,591	1,795,684	1,743,715
BRANDYWINE OPERATING PARTNERSHIP, L.P. | General Partner Capital | As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total partners' equity		1,813,136	1,815,411	1,762,764
BRANDYWINE OPERATING PARTNERSHIP, L.P. | General Partner Capital | Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Total partners' equity		$ (21,545)	$ (19,727)	$ (19,049)