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Reconciliation of Asset Retirement Obligations which are Included in Other Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 6,283	$ 7,364
Liabilities incurred	57	214
Liabilities settled	(236)	(183)
Sale of properties	(1,976)	(1,611)
Accretion expense	385	499
Balance at end of year	$ 4,513	$ 6,283

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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