Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Total Assets	$ 7,862,504	$ 7,752,614
Corporate, Non-Segment [Member]
Total Assets	464,506	416,675
Operating Segments [Member]
Total Assets	7,397,998	7,335,939
Operating Segments [Member] | Industrial [Member]
Total Assets	4,550,731	4,414,740
Operating Segments [Member] | Office [Member]
Total Assets	1,436,477	1,524,501
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,171,828	1,170,420
Operating Segments [Member] | All Other Segments [Member]
Total Assets	78,974	81,056
Operating Segments [Member] | Service Operations [Member]
Total Assets	$ 159,988	$ 145,222