Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Total Segment Assets	$ 6,462,998	$ 6,285,061
Non-Segment Assets - Partnership	605,092	718,921
Total Assets of the Limted Partnership - Segment Repoorting	7,068,090	7,003,982
Non-Segment Assets - General Partner	0	455
Total Assets	7,068,090	7,004,437
Industrial [Member]
Reportable Segment Assets	3,736,149	3,586,250
Office [Member]
Reportable Segment Assets	1,677,805	1,742,196
Medical Office [Member]
Reportable Segment Assets	702,514	580,177
Non-reportable Rental Operations segments [Member]
Reportable Segment Assets	186,864	209,056
Service Operations [Member]
Reportable Segment Assets	$ 159,666	$ 167,382