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Summary of Significant Accounting Policies Asset Retirement Obligation (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement obligation, beginning balance	$ 24.8
Accretion expense	0.9
Revisions in estimated cash flows	26.7
Asset retirement obligation, ending balance	$ 52.4

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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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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