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Fair Value Measurements Fair Value Measurement (Details 2) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 A | Mar. 31, 2014 | Dec. 31, 2014
Liabiliity measured at fair value using Level 3 inputs |  |  |  |  |
Derivative Asset, Fair Value, Gross Asset | $ 585,474,000 us-gaap_DerivativeFairValueOfDerivativeAsset |  |  | $ 592,208,000 us-gaap_DerivativeFairValueOfDerivativeAsset |
Net Profits Plan |  |  |  |  |
Number of Periods Used for Price Assumptions of Strip Prices of Liabilities | 5 sm_NumberOfPeriodsUsedForPriceAssumptionsOfStripPricesOfLiabilities |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract] |  |  |  |  |
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent | 27,136,000 us-gaap_DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent |  |  |  |
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent | 22,802,000 us-gaap_DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent |  |  |  |
Proved and Unproved Oil and Gas Properties |  |  |  |  |
Discount Rate Used for Fair Value of Oil and Gas Properties |  |  |  | 12.00%sm_DiscountRateUsedForFairValueOfOilAndGasProperties |
Number of Periods Used for Price Assumptions of Strip Prices of Liabilities | 5 sm_NumberOfPeriodsUsedForPriceAssumptionsOfStripPricesOfLiabilities |  |  |  |
Impairment of Oil and Gas Properties | 55,526,000 us-gaap_ImpairmentOfOilAndGasProperties |  | 0 us-gaap_ImpairmentOfOilAndGasProperties |  |
Abandonment and impairment of unproved properties | 11,627,000 us-gaap_ImpairmentOfLeasehold |  | 2,801,000 us-gaap_ImpairmentOfLeasehold |  |
Proved Oil and Gas Property, Successful Effort Method | 7,006,832,000 us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod |  |  | 7,348,436,000 us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod |
Unproved Oil and Gas Property, Successful Effort Method | 512,461,000 us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod |  |  | 532,498,000 us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod |
Net Profit Plan liability [Member] |  |  |  |  |
Net Profits Plan |  |  |  |  |
Discount Rate Used to Calculate Currently in Payout Liabilities | 12.00%sm_DiscountRateUsedToCalculateCurrentlyInPayoutLiabilities / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Number of Periods Used for Price Assumptions of Strip Prices of Liabilities | 5 sm_NumberOfPeriodsUsedForPriceAssumptionsOfStripPricesOfLiabilities / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Period Used for Price Assumptions of Strip Prices of Liabilities | 1 sm_PeriodUsedforPriceAssumptionsofStripPricesforLiabilities / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Percent Change in Commodity Prices for Sensitivity Analysis | 5.00%sm_PercentChangeinCommodityPricesforSensitivityAnalysis / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Sensitivity Analysis Change in Liability, Due to Change in Commodity Prices | 2,000,000 sm_SensitivityAnalysisChangeInLiabilityDueToChangeInCommodityPrices / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Percent Change in Discount Rate for Sensitivity Analysis | 1.00%sm_PercentIncreaseandDecreaseinDiscountRateforSensitivityAnalysis / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Sensitivity Analysis Decrease in Liability Due to Change in Discount Rate | 1,000,000 sm_SensitivityAnalysisChangeinLiabilityDuetoChangeinDiscountRate / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract] |  |  |  |  |
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent | 27,136,000 us-gaap_DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) | (3,035,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1] |  |  |
Net settlements | (1,299,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1],[2] |  |  |
Transfers in (out) of Level 3 | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent | 22,802,000 us-gaap_DeferredCompensationCashbasedArrangementsLiabilityClassifiedNoncurrent / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
Proved and Unproved Oil and Gas Properties |  |  |  |  |
Number of Periods Used for Price Assumptions of Strip Prices of Liabilities | 5 sm_NumberOfPeriodsUsedForPriceAssumptionsOfStripPricesOfLiabilities / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |
6.625% Senior Notes Due 2019 [Member] |  |  |  |  |
Liabiliity measured at fair value using Level 3 inputs |  |  |  |  |
Long-term Debt, Fair Value | 359,730,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6625SeniorNotesDue2019Member |  |  | 350,018,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6625SeniorNotesDue2019Member |
6.50% Senior Notes Due 2021 [Member] |  |  |  |  |
Liabiliity measured at fair value using Level 3 inputs |  |  |  |  |
Long-term Debt, Fair Value | 357,438,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2021Member |  |  | 343,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2021Member |
6.125% Senior Notes Due 2022 [Member] |  |  |  |  |
Liabiliity measured at fair value using Level 3 inputs |  |  |  |  |
Long-term Debt, Fair Value | 601,878,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6.125SeniorNotesDue2022Member |  |  | 556,500,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6.125SeniorNotesDue2022Member |
6.50% Senior Notes Due 2023 [Member] |  |  |  |  |
Liabiliity measured at fair value using Level 3 inputs |  |  |  |  |
Long-term Debt, Fair Value | 408,080,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2023Member |  |  | 379,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2023Member |
5.0% Senior Notes Due 2024 [Member] |  |  |  |  |
Liabiliity measured at fair value using Level 3 inputs |  |  |  |  |
Long-term Debt, Fair Value | 472,500,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A5SeniorNotesDue2024Member |  |  | 435,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A5SeniorNotesDue2024Member |
Mid Continent Divestiture 2015 [Member] |  |  |  |  |
Proved and Unproved Oil and Gas Properties |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal | 30,000,000 us-gaap_DisposalGroupNotDiscontinuedOperationGainLossOnDisposal / us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis = sm_MidContinentDivestiture2015Member |  |  |  |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |  |
Proved and Unproved Oil and Gas Properties |  |  |  |  |
Proved Oil and Gas Property, Successful Effort Method | 48,969,000 us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | [3] |  | 33,423,000 us-gaap_ProvedOilAndGasPropertySuccessfulEffortMethod / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | [4]
Unproved Oil and Gas Property, Successful Effort Method | $ 0 us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember |  |  | $ 0 us-gaap_UnprovedOilAndGasPropertySuccessfulEffortMethod / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember |
[1] | Net changes in the Company’s Net Profits Plan liability are shown in the Change in Net Profits Plan liability line item of the accompanying statements of operations
[2] | Settlements represent cash payments made or accrued under the Net Profits Plan.
[3] | This represents a non-financial asset or liability that is measured at fair value on a nonrecurring basis.
[4] | This represents a non-financial asset that is measured at fair value on a nonrecurring basis.

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- Definition Rate applied to the undiscounted amount of estimated cash flows from proved oil and gas properties to arrive at the present value recorded as of the balance sheet date. + References No definition available. + Details
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- Definition Aggregate carrying value as of the balance sheet date of the liabilities for deferred compensation arrangements payable after one year (or the normal operating cycle, if longer). Represents currently earned compensation under cash arrangements (such as a profit-sharing plan, rabbi trust, and employee contract--excluding equity-based arrangements) that is not actually paid until a later date. + References No definition available. + Details
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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313 + Details
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- Definition The gain (loss) resulting from the sale of a disposal group that is not a discontinued operation. It 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 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2443-110228 + Details
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- Definition Amount of increase (decrease) of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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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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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + 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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