SUMMARY OF QUARTERLY RESULTS (UNAUDITED) (Details) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Dec. 11, 2018 USD ($)	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) property $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares
Quarterly Financial Information [Line Items]
Total revenue		$ 147,039	$ 145,331	$ 144,151	$ 143,896	$ 139,203	$ 134,998	$ 133,786	$ 136,358	$ 580,417	$ 544,345	$ 520,493
Net income (loss)		16,874	6,820	6,252	4,583	121,673	(43,522)	12,876	44,445	34,529	135,472	121,177
Net income (loss) allocated to common equity holders		$ 16,676	$ 6,679	$ 6,112	$ 4,404	$ 120,792	$ (43,260)	$ 12,661	$ 43,956	$ 34,267	$ 134,518	$ 120,173
Basic earnings per Common Share (in dollars per shares) | $ / shares		$ 0.09	$ 0.04	$ 0.03	$ 0.03	$ 0.68	$ (0.24)	$ 0.07	$ 0.25	$ 0.19	$ 0.75	$ 0.65
Diluted earnings (loss) per Common Share (in dollars per shares) | $ / shares		$ 0.09	$ 0.04	$ 0.03	$ 0.03	$ 0.67	$ (0.24)	$ 0.07	$ 0.24	$ 0.19	$ 0.75	$ 0.65
Net gain on real estate venture transactions										$ 11,639	$ 142,233	$ 80,526
Gain on promoted interest in unconsolidated real estate venture										0	28,283	0
DRA Austin Venture
Quarterly Financial Information [Line Items]
Net income (loss) allocated to common equity holders										1,300
Net gain on real estate venture transactions	$ 103,800
Gain on promoted interest in unconsolidated real estate venture	$ 28,300
PJP II, PJP VII, And PJP VII
Quarterly Financial Information [Line Items]
Net gain on real estate venture transactions		$ 8,000
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Quarterly Financial Information [Line Items]
Total revenue		147,039	$ 145,331	$ 144,151	$ 143,896	$ 139,203	$ 134,998	$ 133,786	$ 136,358	580,417	544,345	520,493
Net income (loss)		16,874	6,820	6,252	4,583	121,673	(43,522)	12,876	44,445	$ 34,529	$ 135,472	$ 121,177
Net income (loss) allocated to common equity holders		$ 16,772	$ 6,716	$ 6,146	$ 4,430	$ 121,575	$ (43,622)	$ 12,769	$ 44,326
Basic earnings per Common Share (in dollars per shares) | $ / shares		$ 0.09	$ 0.04	$ 0.03	$ 0.03	$ 0.68	$ (0.24)	$ 0.07	$ 0.25	$ 0.19	$ 0.75	$ 0.65
Diluted earnings (loss) per Common Share (in dollars per shares) | $ / shares		$ 0.09	$ 0.04	$ 0.03	$ 0.03	$ 0.67	$ (0.24)	$ 0.07	$ 0.24	$ 0.19	$ 0.75	$ 0.65
Net gain on real estate venture transactions										$ 11,639	$ 142,233	$ 80,526
Gain on promoted interest in unconsolidated real estate venture										$ 0	$ 28,283	$ 0
BRANDYWINE OPERATING PARTNERSHIP, L.P. | DRA Austin Venture
Quarterly Financial Information [Line Items]
Net gain on real estate venture transactions						$ 103,800
Gain on promoted interest in unconsolidated real estate venture						$ 28,300
BRANDYWINE OPERATING PARTNERSHIP, L.P. | Metropolitan Washington, D.C. - Office
Quarterly Financial Information [Line Items]
Provision for impairment on assets held for sale							$ 56,900
Number of properties contributed to a joint venture | property							8
BRANDYWINE OPERATING PARTNERSHIP, L.P. | PJP II, PJP VII, And PJP VII
Quarterly Financial Information [Line Items]
Net gain on real estate venture transactions		$ 8,000