Accumulated Other Comprehensive Income (Loss) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 437,761	$ 368,391	$ 870,571	$ 662,184
Crude oil and condensate revenues	86,341	70,226	145,485	135,881
Income before income taxes	194,319	148,035	372,249	218,794
Tax benefit (expense)	(75,899)	(58,921)	(146,798)	(86,856)
NET INCOME	118,420	89,114	225,451	131,938
Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Income before income taxes	(22,956)	1,617	(93,731)	16,782
Tax benefit (expense)	9,149	(636)	37,359	(6,601)
NET INCOME	(13,807)	981	(56,372)	10,181
Net gain (loss) on cash flow hedges \| Amount Reclassified from Accumulated Other Comprehensive Income \| Commodity contracts
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	(22,320)	(272)	(92,877)	13,056
Crude oil and condensate revenues	(636)	2,094	(854)	4,136
Postretirement benefits \| Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Amortization of net loss		$ (205)		$ (410)

This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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

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Revenue from natural gas production during the reporting period. Natural gas consists largely of the hydrocarbon methane, and is found in underground formations either by itself or with crude oil.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Revenue from sale of residue oil and condensate, which normally exists in the reservoir as gas but condenses out as a liquid during production.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.

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