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Divestitures and Assets Held for Sale (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 15, 2014	Sep. 24, 2014
Business Acquisition [Line Items]
Proved Oil and Gas Property, Successful Effort Method	$ 7,348,436,000us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod	$ 5,637,462,000us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod
Unproved Oil and Gas Property, Successful Effort Method	532,498,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod	271,100,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod
Asset Retirement Obligations, Noncurrent	(120,867,000)us-gaap_AssetRetirementObligationsNoncurrent	(118,692,000)us-gaap_AssetRetirementObligationsNoncurrent
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	43,858,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment	424,849,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment	55,375,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment
Estimated gain on divestiture of assets	646,000us-gaap_GainLossOnSaleOfOilAndGasProperty	27,974,000us-gaap_GainLossOnSaleOfOilAndGasProperty	(27,018,000)us-gaap_GainLossOnSaleOfOilAndGasProperty
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down	27,600,000us-gaap_DisposalGroupNotDiscontinuedOperationLossGainOnWriteDown
Gain (Loss) on Sale of Proved Property	(646,000)us-gaap_GainLossOnSaleOfProvedProperty	(27,974,000)us-gaap_GainLossOnSaleOfProvedProperty	27,018,000us-gaap_GainLossOnSaleOfProvedProperty
Assets Held-for-sale, Current
Reasonably certain period within which sale will take place (in years)	1sm_AssetsHeldForSaleReasonablyCertainPeriodForSale
Oil and gas properties held for sale	17,891,000us-gaap_DisposalGroupIncludingDiscontinuedOperationLongLivedAssetsCurrent	19,072,000us-gaap_DisposalGroupIncludingDiscontinuedOperationLongLivedAssetsCurrent
Asset Retirement Obligations Assets Held-for-sale Noncurrent	438,000sm_AssetRetirementObligationsAssetsHeldForSaleNoncurrent
Oil and Gas Properties
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Depletion, depreciation, amortization, and asset retirement obligation liability accretion			1,700,000sm_DepletionDepreciationAmortizationAndAssetRetirementObligationLiabilityAccretion / us-gaap_ImpairedLongLivedAssetsHeldAndUsedByTypeAxis = us-gaap_OilAndGasPropertiesMember
Gain (Loss) on Sale of Proved Property			33,900,000us-gaap_GainLossOnSaleOfProvedProperty / us-gaap_ImpairedLongLivedAssetsHeldAndUsedByTypeAxis = us-gaap_OilAndGasPropertiesMember
Gooseneck Prospect Acquisition [Member]
Business Acquisition [Line Items]
Net acres acquired					61,000sm_Netacresacquired / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds				84,836,000us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember	325,230,000us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember
Gooseneck Prospect Acquisition [Member] | Fair Value, Measurements, Nonrecurring [Member]
Business Acquisition [Line Items]
Proved Oil and Gas Property, Successful Effort Method				54,360,000us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	203,493,000us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Unproved Oil and Gas Property, Successful Effort Method				29,469,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	126,622,000us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Oil and Gas Property, Successful Effort Method, Net				83,829,000us-gaap_OilAndGasPropertySuccessfulEffortMethodNet / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	330,115,000us-gaap_OilAndGasPropertySuccessfulEffortMethodNet / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Working Capital Acquired				2,625,000sm_WorkingCapitalAcquired / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	2,772,000sm_WorkingCapitalAcquired / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Asset Retirement Obligations, Noncurrent				(1,618,000)us-gaap_AssetRetirementObligationsNoncurrent / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	(2,113,000)us-gaap_AssetRetirementObligationsNoncurrent / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds					325,230,000us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_BusinessAcquisitionAxis = sm_GooseneckProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Gooseneck Second Prospect Acquisition [Member] | Fair Value, Measurements, Nonrecurring [Member]
Business Acquisition [Line Items]
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds				84,836,000us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_BusinessAcquisitionAxis = sm_GooseneckSecondProspectAcquisitionMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Rocky Mountain Acquisition [Domain]
Business Acquisition [Line Items]
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds	134,500,000us-gaap_SignificantAcquisitionsAndDisposalsAcquisitionCostsOrSaleProceeds / us-gaap_BusinessAcquisitionAxis = sm_RockyMountainAcquisition2014Member
Leasehold Acres Consideration to Acquire Unproved Property	7,000sm_LeaseholdAcresConsiderationtoAcquireUnprovedProperty / us-gaap_BusinessAcquisitionAxis = sm_RockyMountainAcquisition2014Member
Rocky Mountain [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	50,100,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_RockyMountainMember
Estimated gain on divestiture of assets	26,900,000us-gaap_GainLossOnSaleOfOilAndGasProperty / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_RockyMountainMember
Mid Continent Divestiture 2013 [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets		368,500,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_MidContinentDivestiture2013Member
Estimated gain on divestiture of assets		25,300,000us-gaap_GainLossOnSaleOfOilAndGasProperty / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_MidContinentDivestiture2013Member
Rocky Mountain Divestiture 2013 [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets		57,100,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_RockyMountainDivestiture2013Member
Estimated gain on divestiture of assets		13,200,000us-gaap_GainLossOnSaleOfOilAndGasProperty / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_RockyMountainDivestiture2013Member
Permian Divestiture 2013 [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets		14,000,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_PermianDivestiture2013Member
Estimated gain on divestiture of assets		(7,000,000)us-gaap_GainLossOnSaleOfOilAndGasProperty / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_PermianDivestiture2013Member
Mid Continent and Rocky Mountain Divestiture 2012 [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets			57,900,000us-gaap_ProceedsFromSaleOfOilAndGasPropertyAndEquipment / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_MidContinentandRockyMountainDivestiture2012Member
Estimated gain on divestiture of assets			$ 7,400,000us-gaap_GainLossOnSaleOfOilAndGasProperty / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_MidContinentandRockyMountainDivestiture2012Member

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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation for an asset held for sale. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Amount classified as long lived assets attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer.

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