Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Total Assets	$ 7,754,839	$ 7,752,614
Operating Segments [Member]
Total Assets	7,389,809	7,335,939
Operating Segments [Member] | Industrial [Member]
Total Assets	4,677,047	4,414,740
Operating Segments [Member] | Office Building [Member]
Total Assets	1,252,627	1,524,501
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,229,632	1,170,420
Operating Segments [Member] | All Other Segments [Member]
Total Assets	71,741	81,056
Operating Segments [Member] | Service Operations [Member]
Total Assets	158,762	145,222
Corporate, Non-Segment [Member]
Total Assets	$ 365,030	$ 416,675