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Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Natural gas and oil properties:    
Properties subject to depletion $ 9,745,129us-gaap_CapitalizedCostsProvedProperties [1] $ 9,624,725us-gaap_CapitalizedCostsProvedProperties [1]
Unproved properties 942,606us-gaap_CapitalizedCostsUnprovedProperties [1] 943,246us-gaap_CapitalizedCostsUnprovedProperties [1]
Total 10,687,735us-gaap_OilAndGasPropertySuccessfulEffortMethodGross [1] 10,567,971us-gaap_OilAndGasPropertySuccessfulEffortMethodGross [1]
Accumulated depletion and depreciation (2,581,753)us-gaap_OilAndGasPropertySuccessfulEffortMethodAccumulatedDepreciationDepletionAndAmortization [1] (2,590,398)us-gaap_OilAndGasPropertySuccessfulEffortMethodAccumulatedDepreciationDepletionAndAmortization [1]
Natural gas and oil properties, successful efforts method, net $ 8,105,982us-gaap_OilAndGasPropertySuccessfulEffortMethodNet [1] $ 7,977,573us-gaap_OilAndGasPropertySuccessfulEffortMethodNet [1]
[1] Includes capitalized asset retirement costs and the associated accumulated amortization.