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Derivative Financial Instruments Gains and Losses (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts |  |  |  |  |  |
Derivatives Settlement Adjustment | $ 41,000,000 sm_DerivativesSettlementAdjustment |  |  |  |  |
Derivative, (Gain) Loss on Derivative, Net [Abstract] |  |  |  |  |  |
Derivative Settlement Gain (Loss) | 12,615,000 sm_DerivativeSettlementGainLoss | [1] | 22,062,000 sm_DerivativeSettlementGainLoss | [1] | 44,264,000 sm_DerivativeSettlementGainLoss | [1]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 583,264,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | [2] | 3,080,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | [2] | 55,630,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | [2]
Amount reclassified from AOCIL | 0 us-gaap_PriceRiskCashFlowHedgeGainOrLossReclassifiedToEarningsNet |  | 1,115,000 us-gaap_PriceRiskCashFlowHedgeGainOrLossReclassifiedToEarningsNet |  | (2,264,000)us-gaap_PriceRiskCashFlowHedgeGainOrLossReclassifiedToEarningsNet |
Gain (Loss) on Hedging Activity |  |  | (1,800,000)us-gaap_GainLossOnOilAndGasHedgingActivity |  | 3,900,000 us-gaap_GainLossOnOilAndGasHedgingActivity |
Crude oil |  |  |  |  |  |
Derivative, (Gain) Loss on Derivative, Net [Abstract] |  |  |  |  |  |
Derivative Settlement Gain (Loss) | 28,410,000 sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_CrudeOilMember |  | (15,161,000)sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_CrudeOilMember |  | (11,893,000)sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_CrudeOilMember |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 457,082,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_CrudeOilMember |  | (14,665,000)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_CrudeOilMember |  | 20,088,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_CrudeOilMember |
Natural Gas [Member] |  |  |  |  |  |
Derivative, (Gain) Loss on Derivative, Net [Abstract] |  |  |  |  |  |
Derivative Settlement Gain (Loss) | (26,706,000)sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasReservesMember |  | 30,338,000 sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasReservesMember |  | 47,270,000 sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasReservesMember |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 93,267,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasReservesMember |  | 14,053,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasReservesMember |  | 15,493,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasReservesMember |
Natural Gas Liquids [Member] |  |  |  |  |  |
Derivative, (Gain) Loss on Derivative, Net [Abstract] |  |  |  |  |  |
Derivative Settlement Gain (Loss) | 10,911,000 sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasLiquidsReservesMember |  | 6,885,000 sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasLiquidsReservesMember |  | 8,887,000 sm_DerivativeSettlementGainLoss / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasLiquidsReservesMember |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | $ 32,915,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasLiquidsReservesMember |  | $ 3,692,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasLiquidsReservesMember |  | $ 20,049,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeByNatureAxis = us-gaap_NaturalGasLiquidsReservesMember |
[1] | (1) Total derivative settlement gain is reported in the derivative cash settlements line item on the accompanying statements of cash flows within net cash provided by operating activities with the change in accrued settlements between years being reported in change in accounts receivable and change in accounts payable and accrued expenses line items. Total derivative settlement gains are adjusted by a $41.0 million decrease attributable to the change in current assets and liabilities at December 31, 2014.
[2] | (2) Total derivative gain is reported in the derivative gain line item on the accompanying statements of cash flows within net cash provided by operating activities.

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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition Gain (loss) incurred in hedging activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5618551-113959 + Details
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