Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Total Assets	$ 7,560,101	$ 7,004,437
Industrial [Member]
Total Assets	3,836,721	3,586,250
Office [Member]
Total Assets	1,683,314	1,742,196
Medical Office [Member]
Total Assets	1,202,929	580,177
All Other Segments [Member]
Total Assets	175,197	209,056
Service Operations [Member]
Total Assets	162,219	167,382
Reportable Segment [Member]
Total Assets	7,060,380	6,285,061
Unallocated Amount to Segment [Member]
Total Assets	0	455
Subsidiary of Common Parent [Member]
Total Assets	7,560,101	7,003,982
Subsidiary of Common Parent [Member]
Total Assets	7,560,101	7,003,982
Subsidiary of Common Parent [Member] | Unallocated Amount to Segment [Member]
Total Assets	$ 499,721	$ 718,921