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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	$ 67.2	$ 63.8
Accretion	0.8	0.7
Liabilities incurred	0.4	0.7
Revisions in estimated cash flows	(6.8)	(1.1)
Liabilities settled	(0.4)	0
AROs at March 31,	$ 61.2	$ 64.1

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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