Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Total Assets	$ 7,802,099	$ 7,560,101
Industrial [Member]
Total Assets	3,835,245	3,836,721
Office [Member]
Total Assets	1,673,827	1,683,314
Medical Office [Member]
Total Assets	1,233,126	1,202,929
All Other Segments [Member]
Total Assets	170,248	175,197
Service Operations [Member]
Total Assets	155,055	162,219
Reportable Segment [Member]
Total Assets	7,067,501	7,060,380
Unallocated Amount to Segment [Member]
Total Assets	$ 734,598	$ 499,721