Segment Reporting (Reportable Segments Consolidated Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Total Assets	$ 6,958,510	$ 6,772,002
Corporate, Non-Segment [Member]
Total Assets	338,746	314,127
Operating Segments [Member]
Total Assets	6,619,764	6,457,875
Operating Segments [Member] | Industrial [Member]
Total Assets	5,042,142	4,828,984
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,365,645	1,338,844
Operating Segments [Member] | All Other Segments [Member]
Total Assets	86,522	162,893
Operating Segments [Member] | Service Operations [Member]
Total Assets	$ 125,455	$ 127,154