Summary Of Significant Accounting Policies (Narrative) (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents			90 days
Interest capitalized	$ 3,200	$ 4,900	$ 7,500	$ 10,200
Compensation costs, leasing and construction activities	14,000	14,100	28,300	27,900
Cash flows from capitalized leasing costs			$ 26,400	21,800
Number of reportable segments | segment			1
Total operating revenues	420,295	401,446	$ 826,904	792,036
Other Expense
Summary Of Significant Accounting Policies [Line Items]
Construction management expense reclassified to other expense		100		300
United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	322,400	305,500	637,100	600,700
Long-lived assets	5,400,000		5,400,000		$ 5,400,000
Outside The United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	97,900	95,900	189,800	191,300
Long-lived assets	2,700,000		2,700,000		2,700,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	54,800	$ 54,500	104,900	$ 109,400
Long-lived assets	$ 1,800,000		$ 1,800,000		$ 1,700,000
Sales
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	13.00%	13.60%	12.70%	13.80%
Long-Lived Assets
Summary Of Significant Accounting Policies [Line Items]
Concentration risk			21.70%		21.30%