Acquisitions and Dispositions (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) buildings	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) buildings	Sep. 30, 2014 USD ($)
Operating Income (Loss)	$ 119,703,000	$ 114,894,000	$ 389,169,000	$ 355,488,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			9 years 2 months 1 day
Operating Leases, Income Statement, Lease Revenue	$ 200,938,000	$ 202,067,000	$ 617,549,000	$ 615,493,000
Industrial [Member]
Number Of Real Estate Properties Acquired | buildings	2		2
Other 2015 Acquisitions [Member]
Operating Income (Loss)			$ 69,000
Operating Leases, Income Statement, Lease Revenue			$ 450,000