Acquisitions and Dispositions (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Proceeds from land and depreciated property sales, net	$ 89,450	$ 498,249
Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	11
Weighted average remaining life of property (years)	8.4
Medical Office Properties [Member] | Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	5
Columbus Ohio [Member] | Industrial Buildings [Member] | Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	2
Chicago Illinois [Member] | Industrial Buildings [Member] | Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	2
Southern California [Member] | Industrial Buildings [Member] | Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	1
Atlanta Georgia [Member] | Industrial Buildings [Member] | Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	1