Asset Retirement Obligation (ARO) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Asset Retirement Obligation [Line Items]
Asset retirement obligations	$ 82,316		$ 59,256
Change in the asset retirement obligation
Beginning balance	64,019		53,443
Accretion expense	5,858		4,812
Revisions in estimated cash flows	10,999		4,465
Ending balance	86,746	[1]	64,019
Nevada Assets
Asset Retirement Obligation [Line Items]
Asset retirement obligations			700
Developmental Activities
Change in the asset retirement obligation
Liabilities incurred	7,249		2,983
Liabilities Settled
Change in the asset retirement obligation
Liabilities settled	(3,325)		(1,803)
Acquisition Activities
Change in the asset retirement obligation
Liabilities incurred	2,651		119
Divestiture Activities
Change in the asset retirement obligation
Liabilities settled	$ (705)		$ 0

[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.