XML 115 R90.htm IDEA: XBRL DOCUMENT

Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation (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 beginning of year	$ 21,284	$ 19,903
Changes in price estimates	(3,696)	(1,071)
Liabilities incurred		859
Accretion expense	1,701	1,593
Balance at end of year	$ 19,289	$ 21,284

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.

+ References+ Details