Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 360,191us-gaap_NaturalGasProductionRevenue	$ 432,809us-gaap_NaturalGasProductionRevenue
Crude oil and condensate revenues	62,558us-gaap_OilAndCondensateRevenue	59,144us-gaap_OilAndCondensateRevenue
Income before income taxes	63,729	177,930
Tax benefit (expense)	(23,474)us-gaap_IncomeTaxExpenseBenefit	(70,899)us-gaap_IncomeTaxExpenseBenefit
Reclassifications to net income	40,255	107,031
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	0	(70,775)
Tax benefit (expense)	0	28,210
Reclassifications to net income	0	(42,565)
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	0	(70,557)
Crude oil and condensate revenues	$ 0	$ (218)

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.

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

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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.

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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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