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Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Detail) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Property, Plant and Equipment [Abstract]    
Proved oil and gas properties $ 5,467,976,000us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod [1] $ 5,156,875,000us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod [1]
Less: Accumulated depreciation, depletion, amortization and impairment (1,157,925,000)us-gaap_OilAndGasPropertySuccessfulEffortMethodAccumulatedDepreciationDepletionAndAmortization (1,043,121,000)us-gaap_OilAndGasPropertySuccessfulEffortMethodAccumulatedDepreciationDepletionAndAmortization
Proved oil and gas properties, net 4,310,051,000oas_ProvedOilAndGasPropertiesNet 4,113,754,000oas_ProvedOilAndGasPropertiesNet
Unproved oil and gas properties 719,662,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod 809,265,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod
Total oil and gas properties, net 5,029,713,000us-gaap_OilAndGasPropertySuccessfulEffortMethodNet 4,923,019,000us-gaap_OilAndGasPropertySuccessfulEffortMethodNet
Other property and equipment 356,940,000us-gaap_PropertyPlantAndEquipmentOther 313,439,000us-gaap_PropertyPlantAndEquipmentOther
Less: Accumulated depreciation (57,291,000)us-gaap_PropertyPlantAndEquipmentOtherAccumulatedDepreciation (49,672,000)us-gaap_PropertyPlantAndEquipmentOtherAccumulatedDepreciation
Other property and equipment, net 299,649,000us-gaap_PropertyPlantAndEquipmentOtherNet 263,767,000us-gaap_PropertyPlantAndEquipmentOtherNet
Total property, plant and equipment, net 5,329,362,000us-gaap_PropertyPlantAndEquipmentNet 5,186,786,000us-gaap_PropertyPlantAndEquipmentNet
Asset retirement costs $ 37,500,000us-gaap_CapitalizedCostsAssetRetirementCosts $ 36,900,000us-gaap_CapitalizedCostsAssetRetirementCosts
[1] Included in the Company’s proved oil and gas properties are estimates of future asset retirement costs of $37.5 million and $36.9 million at March 31, 2015 and December 31, 2014, respectively.