Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 segment	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 United States [Member]	Sep. 30, 2012 United States [Member]	Sep. 30, 2013 United States [Member]	Sep. 30, 2012 United States [Member]	Dec. 31, 2012 United States [Member]	Sep. 30, 2013 Outside The United States [Member]	Sep. 30, 2012 Outside The United States [Member]	Sep. 30, 2013 Outside The United States [Member]	Sep. 30, 2012 Outside The United States [Member]	Dec. 31, 2012 Outside The United States [Member]	Sep. 30, 2013 England [Member]	Sep. 30, 2012 England [Member]	Sep. 30, 2013 England [Member]	Sep. 30, 2012 England [Member]	Dec. 31, 2012 England [Member]	Sep. 30, 2013 Sales [Member] England [Member]	Sep. 30, 2012 Sales [Member] England [Member]	Sep. 30, 2013 Sales [Member] England [Member]	Sep. 30, 2012 Sales [Member] England [Member]	Sep. 30, 2013 Long-Lived Assets [Member] England [Member]	Dec. 31, 2012 Long-Lived Assets [Member] England [Member]
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents			90 days
Number of Reportable Segments			1
Interest capitalized	$ 6,900,000	$ 4,500,000	$ 18,800,000	$ 13,600,000
Long-Lived Assets						5,400,000,000		5,400,000,000		5,000,000,000	2,600,000,000		2,600,000,000		2,500,000,000	1,700,000,000		1,700,000,000		1,700,000,000
Concentration Risk, Percentage																					12.70%	12.60%	13.10%	7.30%	21.20%	22.30%
Compensation costs, leasing and construction activities	9,700,000	8,000,000	29,400,000	23,300,000
Cash flows from capitalized leasing costs			36,400,000	26,500,000
Assets or liabilities for uncertain tax positions	0		0		0
Interest or penalties from uncertain tax positions	0	0	0	0
Asset retirement obligations	1,700,000		1,700,000		1,700,000
Number of reportable segments			1
Total operating revenues	379,456,000	342,479,000	1,101,328,000	929,331,000		294,100,000	270,200,000	845,000,000	778,300,000		85,400,000	72,300,000	256,300,000	151,000,000		48,000,000	43,100,000	144,000,000	67,900,000
Long-lived assets						5,400,000,000		5,400,000,000		5,000,000,000	2,600,000,000		2,600,000,000		2,500,000,000	1,700,000,000		1,700,000,000		1,700,000,000
Concentration risk																					12.70%	12.60%	13.10%	7.30%	21.20%	22.30%
Straight-line rent expense adjustment	10,000,000
Leasehold extension	10 years
Striaght-line rent adjustment run-rate	$ 830,000