Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Total Assets	$ 7,884,195	$ 7,752,614
Corporate, Non-Segment [Member]
Total Assets	425,439	416,675
Operating Segments [Member]
Total Assets	7,458,756	7,335,939
Operating Segments [Member] | Industrial [Member]
Total Assets	4,673,507	4,414,740
Operating Segments [Member] | Office [Member]
Total Assets	1,378,083	1,524,501
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,172,615	1,170,420
Operating Segments [Member] | All Other Segments [Member]
Total Assets	76,736	81,056
Operating Segments [Member] | Service Operations [Member]
Total Assets	$ 157,815	$ 145,222