Asset Retirement Obligation (ARO) (Tables)

12 Months Ended

Dec. 31, 2012

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Year Ended December 31,
(in thousands)	2012			2011
Beginning balance at January 1	$	64,019		$	53,443
Liabilities incurred	7,249			2,983
Liabilities settled	(3,325		)	(1,803		)
Liabilities assumed	2,651			119
Disposition of assets	(705		)	—
Accretion expense	5,858			4,812
Revisions in estimated cash flows	10,999			4,465
Ending balance at December 31(1)	$	86,746		$	64,019

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(1) The asset retirement obligation related to the Nevada Assets, which were held for sale as of December 31, 2011 and sold effective January 31, 2012, was $0.7 million at December 31, 2011 and is included in the asset retirement obligations liability on the December 31, 2011 Balance Sheets. See Note 2 to the Financial Statements.