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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012 Environmental Compliance	Dec. 31, 2010 Legal Reserve
Commitments and Contingencies Disclosure [Line Items]
FirmTransportationLoss	$ 15
FirmTransportationLossUpperRange	18
Reserve established for contingency matters			0.9
Asset retirement obligations		$ 6.4

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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