Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 segment	Mar. 31, 2014	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents	90 days
Interest capitalized	$ 4,300,000	$ 5,300,000
Compensation costs, leasing and construction activities	12,300,000	11,400,000
Cash flows from capitalized leasing costs	13,700,000	9,600,000
Number of reportable segments	1
Total operating revenues	406,609,000	390,590,000
Other Expense
Summary Of Significant Accounting Policies [Line Items]
Construction management expense reclassified to other expense	200,000
United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	314,700,000	295,200,000
Long-lived assets	5,500,000,000		5,400,000,000
Outside The United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	92,000,000	95,400,000
Long-lived assets	2,600,000,000		2,700,000,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	50,000,000	54,900,000
Long-lived assets	$ 1,700,000,000		1,700,000,000
Sales
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	12.30%	14.10%
Long-Lived Assets
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	20.70%		21.30%