Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Total Assets	$ 7,767,363	$ 7,752,614
Corporate, Non-Segment [Member]
Total Assets	414,255	416,675
Operating Segments [Member]
Total Assets	7,353,108	7,335,939
Operating Segments [Member] | Industrial [Member]
Total Assets	4,438,376	4,414,740
Operating Segments [Member] | Office [Member]
Total Assets	1,527,997	1,524,501
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,155,112	1,170,420
Operating Segments [Member] | All Other Segments [Member]
Total Assets	79,899	81,056
Operating Segments [Member] | Service Operations [Member]
Total Assets	$ 151,724	$ 145,222