SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income Taxes (Details) - USD ($) $ in Billions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Contingency [Line Items]
Tax basis of assets, cost for income tax purposes	$ 2.9	$ 3.2
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Income Tax Contingency [Line Items]
Tax basis of assets, cost for income tax purposes	$ 2.9	$ 3.2