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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, 2013
Dec. 31, 2012
Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure [Abstract]      
Present value, beginning balance $ 5,840,368us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1] $ 5,216,337us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1] $ 3,683,515us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Changes during the year      
Revenues less production and other costs (1,271,559)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts (1,234,918)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts (857,848)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts
Net changes in prices, production and other costs 271,566us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts (2,555,391)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts 1,264,736us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts
Development costs incurred 474,275us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod 766,943us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod 575,546us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod
Net changes in future development costs (207,729)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts (45,397)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts 87,080us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts
Extensions and discoveries 1,406,102us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts 2,092,423us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts 1,812,167us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
Revisions of previous quantity estimates (296,418)us-gaap_RevisionsOfPreviousQuantityEstimates (530,755)us-gaap_RevisionsOfPreviousQuantityEstimates (345,965)us-gaap_RevisionsOfPreviousQuantityEstimates
Accretion of discount 711,385us-gaap_AccretionOfDiscount 678,200us-gaap_AccretionOfDiscount 455,501us-gaap_AccretionOfDiscount
Net change in income taxes 477,328us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves 11,433us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves (833,841)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves
Purchases of reserves in-place 1,628us-gaap_IncreaseDueToPurchasesOfMineralsInPlace 1,708,301us-gaap_IncreaseDueToPurchasesOfMineralsInPlace 44,934us-gaap_IncreaseDueToPurchasesOfMineralsInPlace
Sales of reserves in-place (3,172,187)us-gaap_DecreaseDueToSalesOfMineralsInPlace (410,415)us-gaap_DecreaseDueToSalesOfMineralsInPlace (558,257)us-gaap_DecreaseDueToSalesOfMineralsInPlace
Timing differences and other (217,148)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther [2] 143,607us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther [2] (111,231)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther [2]
Net change for the year (1,822,757)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease 624,031us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease 1,532,822us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease
Present value, ending balance $ 4,017,611us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1] $ 5,840,368us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1] $ 5,216,337us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves [1]
[1] Includes approximately $781.6 million, $952.7 million and $932.8 million attributable to noncontrolling interests at December 31, 2013, 2012, and 2011 respectively.
[2] The change in timing differences and other are related to revisions in the Company’s estimated time of production and development.