Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents	90 days
Interest capitalized	$ 5,300,000	$ 5,300,000
Compensation costs, leasing and construction activities	12,400,000	10,100,000
Cash flows from capitalized leasing costs	9,600,000	10,800,000
Number of reportable segments	1
Total operating revenues	390,590,000	358,370,000
Reclassification from rental property operating and maintenance expense to change in fair value of contingent consideration		1,300,000
United States [Member]
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	295,200,000	274,000,000
Long-lived assets	5,700,000,000		5,600,000,000
Outside The United States [Member]
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	95,400,000	84,400,000
Long-lived assets	2,700,000,000		2,700,000,000
United Kingdom [Member]
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	54,900,000	47,300,000
Long-lived assets	$ 1,700,000,000		$ 1,800,000,000
Sales [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	14.10%	13.20%