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Summary of Significant Accounting Policies Other (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 Years | Dec. 31, 2013 | Dec. 31, 2012
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |
Restricted Cash and Cash Equivalents | $ 0 us-gaap_RestrictedCashAndCashEquivalents |  |
Revolving credit facility | 166,000,000 us-gaap_LineOfCredit | 0 us-gaap_LineOfCredit |
Allowance for Doubtful Accounts Receivable | 0 us-gaap_AllowanceForDoubtfulAccountsReceivable | 0 us-gaap_AllowanceForDoubtfulAccountsReceivable |
Capitalization of Development and Implementation of Internal Use Software | 35,000,000 sm_CapitalizationofDevelopmentandImplementationofInternalUseSoftware | 11,200,000 sm_CapitalizationofDevelopmentandImplementationofInternalUseSoftware |
Concentration of Credit Risk and Major Customers [Abstract] |  |  |
Number of Major Customers | 1 sm_NumberOfMajorCustomers | 3 sm_NumberOfMajorCustomers | 2 sm_NumberOfMajorCustomers
Marketing agreement length for gathering, transportation, and processing through-put commitments | 10 sm_Marketingagreementlengthforgatheringtransportationandprocessingthroughputcommitments |  |
Number of Derivative Counterparties | 9 sm_NumberOfDerivativeCounterparties |  |
Property, Plant and Equipment [Abstract] |  |  |
Property, Plant, and Equipment, Salvage Value | 50,800,000 us-gaap_PropertyPlantAndEquipmentSalvageValue | 57,500,000 us-gaap_PropertyPlantAndEquipmentSalvageValue |
Inventory, Net [Abstract] |  |  |
Property, Plant and Equipment, Estimated Useful Lives | three to 30 years |  |
Revenues [Abstract] |  |  |
Revenue Receipt, Days After Sale, Low End of Range | 30 sm_RevenueReceiptDaysAfterSaleLowEndOfRange |  |
Revenue Receipt, Days After Sale, High End of Range | 90 sm_RevenueReceiptDaysAfterSaleHighEndOfRange |  |
Impairment of Proved Properties [Abstract] |  |  |
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast | 5 sm_PeriodOfNewYorkMercantileExchangeStripPricingUsedForPriceForecast |  |
Impairment of proved properties | 84,480,000 us-gaap_ImpairmentOfOilAndGasProperties | 172,641,000 us-gaap_ImpairmentOfOilAndGasProperties | 208,923,000 us-gaap_ImpairmentOfOilAndGasProperties
Abandonment and impairment of unproved properties | 75,638,000 us-gaap_ImpairmentOfLeasehold | 46,105,000 us-gaap_ImpairmentOfLeasehold | 16,342,000 us-gaap_ImpairmentOfLeasehold
Regency Gas Services, LLC [Member] |  |  |
Concentration of Credit Risk and Major Customers [Abstract] |  |  |
Entity-Wide Revenue, Major Customer, Percentage |  | 26.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_MajorCustomerTwoMember | 21.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_MajorCustomerTwoMember
Anadarko Petroleum Company [Member] |  |  |
Concentration of Credit Risk and Major Customers [Abstract] |  |  |
Entity-Wide Revenue, Major Customer, Percentage | 19.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_MajorCustomerOneMember | 16.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_MajorCustomerOneMember |
Plains Marketing, LP [Member] |  |  |
Concentration of Credit Risk and Major Customers [Abstract] |  |  |
Entity-Wide Revenue, Major Customer, Percentage |  | 12.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_MajorCustomerThreeMember | 13.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_MajorCustomerThreeMember
Unnamed Major Customer with Related Entities [Member] |  |  |
Concentration of Credit Risk and Major Customers [Abstract] |  |  |
Entity-Wide Revenue, Major Customer, Percentage | 14.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_MajorCustomersAxis = sm_UnnamedMajorCustomerwithRelatedEntitiesMember |  |
Number of Entities Related to One Unnamed Customer | 4 sm_NumberofEntitiesRelatedtoOneUnnamedCustomer / us-gaap_MajorCustomersAxis = sm_UnnamedMajorCustomerwithRelatedEntitiesMember |  |
Maximum Percentage of Total Production Revenue for Individual Entity | 10.00%sm_MaximumPercentageofTotalProductionRevenueforIndividualEntity / us-gaap_MajorCustomersAxis = sm_UnnamedMajorCustomerwithRelatedEntitiesMember |  |
Convertible Debt [Member] |  |  |
Debt Instruments [Abstract] |  |  |
Debt Instrument, Convertible, Conversion Price |  |  | $ 54.42 us-gaap_DebtInstrumentConvertibleConversionPrice1 / us-gaap_DebtInstrumentAxis = us-gaap_ConvertibleDebtMember
Debt Instrument, Principal Amount Denominator for Conversion into Common Stock |  |  | $ 1,000 sm_DebtInstrumentPrincipalAmountDenominatorForConversionIntoCommonStock / us-gaap_DebtInstrumentAxis = us-gaap_ConvertibleDebtMember
Convertible Debt [Member] | Convertible Debt [Member] |  |  |
Debt Instruments [Abstract] |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  | 3.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = us-gaap_ConvertibleDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_ConvertibleDebtMember
Debt instrument convertible conversion rate |  |  | 864,106 us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities / us-gaap_DebtInstrumentAxis = us-gaap_ConvertibleDebtMember / us-gaap_LongtermDebtTypeAxis = us-gaap_ConvertibleDebtMember

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- Definition The adjustment to reduce the value of existing agreements that specify the lessee's rights to use the leased property. This expense is charged when the estimates of future profits generated by the leased property are reduced. + 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 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228 + Details
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