Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Total Assets	$ 7,886,839	$ 7,560,101
Corporate, Non-Segment [Member]
Total Assets	411,119	499,721
Operating Segments [Member]
Total Assets	7,475,720	7,060,380
Operating Segments [Member] | Industrial [Member]
Total Assets	4,293,447	3,836,721
Operating Segments [Member] | Office [Member]
Total Assets	1,672,180	1,683,314
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,270,802	1,202,929
Operating Segments [Member] | All Other Segments [Member]
Total Assets	84,619	175,197
Operating Segments [Member] | Service Operations [Member]
Total Assets	$ 154,672	$ 162,219