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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
ARO Liability [Roll Forward]
ARO liability, Beginning Balance	$ 163.9	$ 148.3
Accretion	5.1	4.8
Liabilities incurred	3.6	2.9
Liabilities settled	(0.4)	(0.3)
ARO liability, Ending Balnce	$ 172.2	$ 155.7

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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