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Changes in Asset Retirement Obligations (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Asset Retirement Obligations [Line Items]
Asset retirement obligations as of the beginning of the period, including current portion			$ 425,800,000
Liabilities settled			(47,200,000)
Liabilities assumed			14,500,000
Divestment of properties			(7,600,000)
Accretion expense	(8,405,000)	(7,700,000)	(24,926,000)	(23,134,000)
Asset retirement obligations as of the end of the period, including current portion	$ 410,400,000		$ 410,400,000

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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