BENEFICIARIES' EQUITY OF THE PARENT COMPANY - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 08, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 03, 2019	Jan. 10, 2017
Class of Stock [Line Items]
Dividends, common stock	$ 32,700
Preferred shares authorized (in shares)		20,000,000	20,000,000
Preferred shares outstanding (in shares)		0	0
Repurchased and retired (in shares)		6,248,483	1,337,169	1,729,278	0
Share price (in usd per share)		$ 9.60	$ 12.92	$ 12.64
Repurchased and retired, value		$ 59,999	$ 17,281	$ 21,858
Common stock, par or stated value per share (usd per share)		$ 0.01	$ 0.01
Common stock, shares authorized (in shares)		400,000,000	400,000,000
Cash proceeds, net of issuance costs		$ 0	$ 0	$ 416
Common shares remain available for issuance (in shares)		0
Offering Program
Class of Stock [Line Items]
Common stock, shares authorized (in shares)							16,000,000
Common shares issued under offering program (in shares)		0	0	23,311
Weighted average price per share (usd per share)				$ 18.04
Cash proceeds, net of issuance costs				$ 400
Maximum
Class of Stock [Line Items]
Authorized amount						$ 150,000
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Class of Stock [Line Items]
Repurchase program, number of common units retired for each common share repurchased (in units)		1
Cash proceeds, net of issuance costs		$ 0	$ 0	$ 416
Dividend Declared
Class of Stock [Line Items]
Dividends payable, amount per share (in usd per share)	$ 0.19
Redeemable Common Limited Partnership Units
Class of Stock [Line Items]
Antidilutive securities excluded from computation of earnings per share (in units)		981,634	981,634	982,871