Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Total Segment Assets	$ 6,964,588	$ 7,192,284
Non-Segment Assets	540,073	451,992
Assets	7,504,661	7,644,276
Office [Member]
Total Segment Assets	2,759,739	3,122,565
Industrial [Member]
Total Segment Assets	3,388,125	3,210,566
Non-reportable Rental Operations segments [Member]
Total Segment Assets	640,885	627,491
Service Operations [Member]
Total Segment Assets	$ 175,839	$ 231,662