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Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy
Balance at beginning of period	$ 195,127	$ 14,746	$ 112,307
Total Gains or (Losses) (Realized or Unrealized):
Included in Earnings	224,614	85,375	172,764
Included in Other Comprehensive Income	(157,478)	181,346	(97,335)
Settlements	(221,489)	(86,340)	(172,990)
Transfers In and/or Out of Level 3	385
Balance at end of period	41,159	195,127	14,746
Natural Gas Revenues
Derivative disclosures
Unrealized gain (loss) included in Natural Gas Revenues	$ 500	$ 1,000	$ 200

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- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

+ References

No definition available.

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- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in other comprehensive income (a separate component of shareholders' equity).

+ References

No definition available.

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- Definition

Settlements that have taken place during the period in relation to assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

No definition available.

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- Definition

This element represents [net] transfers in to and out of assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ References

No definition available.

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- Definition

This element represents the net asset (liability) measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

No definition available.

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- Definition

Net gain (loss) from the increase (decrease) in fair value of derivative instrument not designated as hedging instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph b(1)(b)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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