Segment Reporting (Reportable Segments Consolidated Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Total Assets	$ 6,772,002	$ 6,895,515
Operating Segments [Member]
Total Assets	6,457,875	6,326,379
Operating Segments [Member] | Industrial [Member]
Total Assets	4,828,984	4,552,107
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,338,844	1,269,546
Operating Segments [Member] | All Other Segments [Member]
Total Assets	162,893	367,469
Operating Segments [Member] | Service Operations [Member]
Total Assets	127,154	137,257
Corporate, Non-Segment [Member]
Total Assets	$ 314,127	$ 569,136