Accumulated Other Comprehensive Income / (Loss) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2014	Mar. 31, 2013
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 432,809	$ 293,793
Crude oil and condensate revenues	59,144	65,655
Income before income taxes	177,930	70,759
Tax (expense) / benefit	(70,899)	(27,935)
NET INCOME	107,031	42,824
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	(70,775)	15,165
Tax (expense) / benefit	28,210	(5,965)
NET INCOME	(42,565)	9,200
Net gains / (losses) 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	(70,557)	13,328
Crude oil and condensate revenues	(218)	2,042
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)

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)

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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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