Volumetric Production Payments (Deferred Revenue Carrying Values) (Details) (Volumetric Production Payment [Member], USD $) In Thousands	6 Months Ended
Jun. 30, 2011
Volumetric Production Payment [Member]
Deferred revenue at December 31, 2010	$ 87,020
Less: 2011 amortization	(22,290)
Deferred revenue at June 30, 2011	$ 64,730

Amount of previously reported deferred or unearned revenue that was recognized as revenue during the period. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 13
-Section A

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 13.A.4(a).Q1)
-URI http://asc.fasb.org/extlink&oid=6600647&loc=d3e214044-122780

Name:	us-gaap_DeferredRevenueRevenueRecognized
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration