Summary of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Summary Of Significant Accounting Policies [Abstract]
Period in which short-term investment become cash equivalents (days)	90
Assets or liabilities for uncertain tax positions	$ 0		$ 0
Interest or penalties from uncertain tax positions	0	0
Asset retirement obligations	1,200,000		1,200,000
Interest capitalized	4,500,000	4,700,000
Compensation costs, leasing and construction activities	7,900,000	6,200,000
Cash flows from capitalized leasing costs	$ 6,200,000	$ 4,900,000

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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This element represents the total of interest expense recognized for an underpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity and the amount of statutory penalties in the period in which the entity claims or expects to claim a tax position, in its tax return, that does not meet the minimum statutory threshold to avoid payment of penalties.

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