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

X
- Definition 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. + References No definition available. + Details
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- Definition Represents the period where there is reasonable certainty that the sale will take place for the assets to be classified as held for sale. + References No definition available. + Details
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- Definition The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets. Amount of accretion expense recognized in the income statement during the period that is associated with asset retirement obligations. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. Such accretion expense representing changes in the liability for an asset retirement obligation due to passage of time is the result of applying an interest method of allocation to the amount of the liability at the beginning of the period. The interest rate used to measure that change shall be the credit-adjusted risk-free rate that existed when the liability, or portion thereof, was initially measured. That amount shall be recognized as an increase in the carrying amount of the liability and as an expense classified as an operating item in the statement of income. Accretion expense shall not be considered to be interest cost for purposes of capitalization of interest. + References No definition available. + Details
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- Definition Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1107-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1436-107760 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2473-110228 + Details
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- Definition The loss (gain) resulting from the write-down of the carrying value of assets classified as a disposal group, but one which is not further classified as a discontinued operation, to fair value less cost to sell. A (gain) results from a subsequent increase in fair value less cost to sell, but not in excess of the cumulative loss previously recognized. For example, the write-down of a disposal group subject to a plan of sale, which was not sold in the current period, to its held-for-sale value. The loss (gain) is included in income from continuing operations before income taxes in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
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- Definition Amount of gain (loss) on sale or disposal of proven and unproven oil and gas properties. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition Amount of gain (loss) on sale or disposal of property with proven oil and gas reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition Oil and gas properties net of accumulated depreciation, depletion, amortization, impairment, and abandonment, carried under the successful effort method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-10.(b)) -URI http://asc.fasb.org/extlink&oid=27015464&loc=d3e511914-122862 + Details
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- Definition The cash inflow to dispose of long-lived, physical assets and mineral interests in oil and gas properties used for normal oil and gas operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
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- Definition Oil and Gas properties with proved reserves under the successful effort method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Glossary Proved Properties -URI http://asc.fasb.org/extlink&oid=6522428 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6474861&loc=d3e64746-109465 + Details
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- Definition Oil and Gas properties with unproved reserves under the successful effort method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61926-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -Glossary Unproved Properties -URI http://asc.fasb.org/extlink&oid=6527831 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 360 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6474861&loc=d3e64746-109465 + Details
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