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Changes in Asset Retirement Obligations (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Asset Retirement Obligation [Line Items]
Asset retirement obligations as of the beginning of the period, including current portion	$ 488,300,000
Liabilities settled	(14,900,000)
Accretion expense	8,263,000	8,266,000
Asset retirement obligations as of the end of the period, including current portion	$ 481,700,000

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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