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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	$ 63.8	$ 60.9
Accretion	0.7	0.7
Liabilities incurred	0.7	0.2
Revisions	(1.1)	2.5
Liabilities settled	0	(0.1)
AROs at end of period	$ 64.1	$ 64.2

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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