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Supplemental Information on Oil and Natural Gas Producing Activities - Estimate of Changes in Standardized Measure of Discounted Future Net Cash Flows from Proved Reserves (Unaudited) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Extractive Industries [Abstract]      
Present value, beginning balance $ 4,017,611us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1],[2] $ 5,840,368us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1],[2] $ 5,216,337us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Changes during the year      
Revenues less production and other costs (1,043,060)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts (1,271,559)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts (1,234,918)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts
Net changes in prices, production and other costs 331,694us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts 271,566us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts (2,555,391)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts
Development costs incurred 364,262us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod 474,275us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod 766,943us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod
Net changes in future development costs (341,183)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts (207,729)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts (45,397)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts
Extensions and discoveries 1,785,963us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts 1,406,102us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts 2,092,423us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
Revisions of previous quantity estimates (77,688)us-gaap_RevisionsOfPreviousQuantityEstimates (296,418)us-gaap_RevisionsOfPreviousQuantityEstimates (530,755)us-gaap_RevisionsOfPreviousQuantityEstimates
Accretion of discount 477,458us-gaap_AccretionOfDiscount 711,385us-gaap_AccretionOfDiscount 678,200us-gaap_AccretionOfDiscount
Net change in income taxes (256,371)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves 477,328us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves 11,433us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves
Purchases of reserves in-place 50,958us-gaap_IncreaseDueToPurchasesOfMineralsInPlace 1,628us-gaap_IncreaseDueToPurchasesOfMineralsInPlace 1,708,301us-gaap_IncreaseDueToPurchasesOfMineralsInPlace
Sales of reserves in-place (1,058,330)us-gaap_DecreaseDueToSalesOfMineralsInPlace (3,172,187)us-gaap_DecreaseDueToSalesOfMineralsInPlace (410,415)us-gaap_DecreaseDueToSalesOfMineralsInPlace
Timing differences and other (163,562)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther [3] (217,148)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther [3] 143,607us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther [3]
Net change for the year 70,141us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease (1,822,757)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease 624,031us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease
Present value, ending balance $ 4,087,752us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1],[2] $ 4,017,611us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1],[2] $ 5,840,368us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1],[2]
[1] Includes approximately $643.3 million, $781.6 million and $952.7 million attributable to noncontrolling interests at December 31, 2014, 2013 and 2012 respectively.
[2] Includes approximately $643.3 million, $781.6 million and $952.7 million attributable to noncontrolling interests at December 31, 2014, 2013, and 2012 respectively.
[3] The change in timing differences and other are related to revisions in the Company’s estimated time of production and development.