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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
Reconciliation of Changes in Asset Retirement Obligations [Line Items]
Balance at January 1	$ 43,233	$ 41,079
Accretion expense	2,712	2,645
Liabilities incurred	204	3,062
Liabilities settled	(126)	(2,091)
Revisions in estimated cash flows	150	(1,462)
Balance at December 31	$ 46,173	$ 43,233

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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