Asset Retirement Obligation (ARO) (Tables)

12 Months Ended

Dec. 31, 2011

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Year Ended December 31,
(in thousands)	2011			2010
Beginning balance at January 1	$	53,443		$	43,487
Liabilities incurred	2,983			3,721
Liabilities settled	(1,803		)	(1,832		)
Liabilities assumed	119			3,498
Accretion expense	4,812			4,569
Revisions in estimated cash flows	4,465			—
Ending balance at December 31(1)	$	64,019		$	53,443

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(1) The asset retirement obligation related to the Nevada Assets, which were held for sale as of December 31, 2011, 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 3 to the Financial Statements.