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Fair Value Measurements (Details 2) (USD $) | 12 Months Ended
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Dec. 31, 2014 A | Dec. 31, 2013 | Dec. 31, 2012
Proved Oil and Gas Properties |  |  |  |  |  |
Discount rate used for fair value of oil and gas properties (as a percent) | 12.00%sm_DiscountRateUsedForFairValueOfOilAndGasProperties |  |  |  |  |
Number of periods used for price assumptions of strip prices of liabilities | 5 sm_NumberOfPeriodsUsedForPriceAssumptionsOfStripPricesOfLiabilities |  |  |  |  |
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 |  |  |
Net Profit Plan liability [Member] |  |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Discount rate to calculate liability pools currently in payout (as a percent) | 12.00%sm_DiscountRateUsedToCalculateCurrentlyInPayoutLiabilities / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |  |
Period used for price assumptions of strip prices of liabilities (in years) | 1 sm_PeriodUsedForPriceAssumptionsOfStripPricesOfLiabilities / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |  |
Percentage change in commodity prices (as a percent) | 5.00%sm_PercentChangeinCommodityPricesforSensitivityAnalysis / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |  |
Change in liability due to change in commodity prices by 5 percent | 2,600,000 sm_SensitivityAnalysisChangeInLiabilityDueToChangeInCommodityPrices / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |  |
Percent Change in Discount Rate for Sensitivity Analysis | 1.00%sm_PercentDecreaseInDiscountRateForSensitivityAnalysis / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |  |
Sensitivity Analysis Change in Liability Due to Change in Discount Rate | 1,100,000 sm_SensitivityAnalysisChangeinLiabilityDuetoChangeinDiscountRate / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Net Profits Plan Liability: Beginning Balance | 56,985,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 78,827,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 107,731,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |
Net increase (decrease) in liability | (12,492,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1] | 3,527,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1] | (9,251,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1]
Net settlements | (17,357,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1],[2] | (25,369,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember | [1],[2] | (19,653,000)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 |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |
Net Profits Plan Liability: Ending balance | 27,136,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 56,985,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 78,827,000 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |
Cash Payments Made or Accrued under Profit Sharing Plan Related to Divested Property | 8,300,000 sm_CashPaymentsMadeOrAccruedUnderProfitSharingPlanRelatedToDivestedProperty / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 10,300,000 sm_CashPaymentsMadeOrAccruedUnderProfitSharingPlanRelatedToDivestedProperty / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |  | 2,300,000 sm_CashPaymentsMadeOrAccruedUnderProfitSharingPlanRelatedToDivestedProperty / us-gaap_FairValueByLiabilityClassAxis = sm_NetProfitPlanLiabilityMember |
6.625% Senior Notes Due 2019 [Member] |  |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Long-term Debt, Fair Value | 350,018,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6625SeniorNotesDue2019Member |  | 374,290,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6625SeniorNotesDue2019Member |  |  |
6.50% Senior Notes Due 2021 [Member] |  |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Long-term Debt, Fair Value | 343,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2021Member |  | 373,625,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2021Member |  |  |
6.125% Senior Notes Due 2022 [Member] |  |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Long-term Debt, Fair Value | 556,500,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6.125SeniorNotesDue2022Member | [3] | 0 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A6.125SeniorNotesDue2022Member | [3] |  |
6.50% Senior Notes Due 2023 [Member] |  |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Long-term Debt, Fair Value | 379,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2023Member |  | 422,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A650SeniorNotesDue2023Member |  |  |
5% Senior Notes Due 2024 [Member] |  |  |  |  |  |
Liability measured at fair value using Level 3 inputs |  |  |  |  |  |
Long-term Debt, Fair Value | $ 435,000,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A5SeniorNotesDue2024Member |  | $ 475,315,000 us-gaap_LongTermDebtFairValue / us-gaap_DebtInstrumentAxis = sm_A5SeniorNotesDue2024Member |  |  |
[1] | (1)Net changes in the Net Profits Plan liability are shown in the Change in Net Profits Plan liability line item of the accompanying statements of operations.
[2] | (2)Settlements represent cash payments made or accrued under the Net Profits Plan. The amounts in the table include cash payments made or accrued under the Net Profits Plan of $8.3 million, $10.3 million, and $2.3 million relating to divestiture proceeds for the years ended December 31, 2014, 2013, and 2012, respectively.
[3] | (1) The 2022 Notes were issued on November 17, 2014.

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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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