Acquisitions and Dispositions (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Proceeds from land and depreciated property sales, net	$ 63,281	$ 437,494
Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	7
Weighted average remaining life of property (years)	8.7
Medical Office Properties [Member] | Other 2012 Acquisitions [Member] [Domain]
Number of real estate properties acquired	4
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	1