Acquisitions and Dispositions (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) buildings	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) buildings	Jun. 30, 2014 USD ($)
Operating Income (Loss)	$ 181,266,000	$ 181,220,000	$ 269,466,000	$ 240,289,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			9 years 2 months 01 day
Operating Leases, Income Statement, Lease Revenue	$ 201,996,000	$ 204,780,000	$ 416,611,000	$ 413,426,000
Industrial [Member]
Number Of Real Estate Properties Acquired | buildings	1		1
Other 2015 Acquisitions [Member]
Operating Income (Loss)			$ 57,000
Operating Leases, Income Statement, Lease Revenue			$ 77,000