Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Total Segment Assets	$ 6,357,526	$ 6,285,061
Non-Segment Assets	514,879	719,376
Assets	6,872,405	7,004,437
Industrial [Member]
Reportable Segment Assets	3,634,930	3,586,250
Office [Member]
Reportable Segment Assets	1,683,691	1,742,196
Medical Office [Member]
Reportable Segment Assets	674,298	580,177
Non-reportable Rental Operations segments [Member]
Reportable Segment Assets	202,139	209,056
Service Operations [Member]
Reportable Segment Assets	$ 162,468	$ 167,382