Segment Reporting (Reportable Segments Consolidated Assets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Total Assets	$ 7,730,504	$ 7,754,839
Corporate, Non-Segment [Member]
Total Assets	456,267	365,030
Operating Segments [Member]
Total Assets	7,274,237	7,389,809
Operating Segments [Member] | Industrial [Member]
Total Assets	4,672,992	4,677,047
Operating Segments [Member] | Office [Member]
Total Assets	1,212,796	1,252,627
Operating Segments [Member] | Medical Office [Member]
Total Assets	1,210,170	1,229,632
Operating Segments [Member] | All Other Segments [Member]
Total Assets	22,909	71,741
Operating Segments [Member] | Service Operations [Member]
Total Assets	$ 155,370	$ 158,762