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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation [Abstract]
Asset Retirement Obligation, Current	$ 1.7
ARO Liability [Roll Forward]
ARO liability, Beginning Balance	163.9	148.3
Accretion	10.5	9.7
Liabilities incurred	8.5	7.9
Revisions of Estimates	11.1	0
Liabilities settled	(0.9)	(2.0)
ARO liability, Ending Balance	$ 193.1	$ 163.9

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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