Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Total Assets	$ 7,190,117	$ 7,004,437
Industrial [Member]
Total Assets	3,753,022	3,586,250
Office [Member]
Total Assets	1,676,389	1,742,196
Medical Office [Member]
Total Assets	820,898	580,177
All Other Segments [Member]
Total Assets	180,998	209,056
Service Operations [Member]
Total Assets	161,953	167,382
Reportable Segment [Member]
Total Assets	6,593,260	6,285,061
Unallocated Amount to Segment [Member]
Total Assets	0	455
Subsidiary of Common Parent [Member]
Total Assets	7,190,117	7,003,982
Subsidiary of Common Parent [Member]
Total Assets	7,190,117	7,003,982
Subsidiary of Common Parent [Member] | Unallocated Amount to Segment [Member]
Total Assets	$ 596,857	$ 718,921