Asset Retirement Obligation (ARO) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Change in the asset retirement obligation
Beginning balance	$ 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		0
Ending balance	64,019	[1]	53,443
Nevada Assets
Change in the asset retirement obligation
Ending balance	$ 700

[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.