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Changes in Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation [Line Items]
Asset retirement obligations as of the beginning of the year, including current portion	$ 425,779	$ 373,920	$ 320,599
Liabilities incurred	3,869	7,993	1,528
Liabilities settled	(67,641)	(63,226)	(36,392)
Liabilities assumed	15,263	59,441
Divestment of properties	(7,563)	(10,900)	(692)
Accretion expense	33,331	30,764	34,469
Revision of estimates	85,264	27,787	54,408
Asset retirement obligations as of the end of the year, including current portion	$ 488,302	$ 425,779	$ 373,920

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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