Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			3 Months Ended			12 Months Ended
	Mar. 31, 2013 segment	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 United States [Member]	Mar. 31, 2012 United States [Member]	Dec. 31, 2012 United States [Member]	Mar. 31, 2013 Outside The United States [Member]	Mar. 31, 2012 Outside The United States [Member]	Dec. 31, 2012 Outside The United States [Member]	Mar. 31, 2013 England [Member]	Mar. 31, 2012 England [Member]	Dec. 31, 2012 England [Member]	Mar. 31, 2013 Sales [Member] England [Member]	Mar. 31, 2012 Sales [Member] England [Member]	Mar. 31, 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
Interest capitalized	$ 5,300,000	$ 4,500,000
Compensation costs, leasing and construction activities	10,100,000	7,900,000
Cash flows from capitalized leasing costs	10,800,000	6,200,000
Assets or liabilities for uncertain tax positions	0		0
Interest or penalties from uncertain tax positions	0	0
Asset retirement obligations	1,800,000		1,700,000
Number of reportable segments	1
Total operating revenues	358,370,000	283,148,000		274,000,000	246,500,000		84,400,000	36,600,000		47,300,000	12,200,000
Long-lived assets				$ 5,200,000,000		$ 5,000,000,000	$ 2,500,000,000		$ 2,500,000,000	$ 1,600,000,000		$ 1,700,000,000
Concentration risk													13.20%	4.30%	20.70%	22.30%