Summary of Significant Accounting Policies (Income Taxes) (Details) (USD $) In Billions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Income Tax Contingency [Line Items]
Distributable income minimum percentage	90.00%
Tax basis of assets	$ 4.5	$ 4.5
Amount of Federal excise tax if sufficient taxable income is not distributed within prescribed time limits	4.00%
Excise tax annual amount, if certain requirements are not meant	4.00%
Excise tax, if sum of ordinary income is less than	85.00%
Excise tax, if Net capital gain exceeds cash distributions and certain taxes paid	95.00%
BRANDYWINE OPERATING PARTNERSHIP, L.P.
Income Tax Contingency [Line Items]
Tax basis of assets	$ 4.5	$ 4.5
Amount of Federal excise tax if sufficient taxable income is not distributed within prescribed time limits	4.00%
Excise tax annual amount, if certain requirements are not meant	4.00%
Excise tax, if sum of ordinary income is less than	85.00%
Excise tax, if Net capital gain exceeds cash distributions and certain taxes paid	95.00%