Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
TotalSegmentAssets	$ 6,285,061	$ 7,192,284
Non-Segment Assets	719,376	451,992
Assets, total	7,004,437	7,644,276
Office [Member]
Total Segment Assets	1,742,196	3,122,565
Industrial [Member]
Total Segment Assets	3,586,250	3,210,566
Non-reportable Rental Operations segments [Member]
Total Segment Assets	789,233	627,491
Service Operations [Member]
Total Segment Assets	$ 167,382	$ 231,662