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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, 2011	Dec. 31, 2010
Reconciliation of Changes in Asset Retirement Obligations [Line Items]
Balance at beginning of year	$ 7,364	$ 6,835
Liabilities incurred	214	126
Liabilities settled	(183)	(41)
Sale of properties	(1,611)
Accretion expense	499	444
Balance at end of year	$ 6,283	$ 7,364

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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