Accumulated Other Comprehensive Income (Loss) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 347,970	$ 341,901	$ 1,218,540	$ 1,004,085
Crude oil and condensate revenues	82,563	84,209	228,047	220,090
Income before income taxes	172,919	115,736	545,167	334,530
Tax benefit (expense)	(72,131)	(45,847)	(218,928)	(132,703)
Reclassifications to net income	100,788	69,889	326,239	201,827
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	(21,557)	19,568	(115,288)	36,351
Tax benefit (expense)	8,592	(7,696)	45,951	(14,298)
Reclassifications to net income	(12,965)	11,872	(69,337)	22,053
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	(21,427)	20,766	(114,304)	33,822
Crude oil and condensate revenues	(130)	(1,082)	(984)	3,054
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	$ 0	$ (116)	$ 0	$ (525)

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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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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