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Income Taxes (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
IRS Proposed R&D Tax Adjustment $ 4,600,000sm_IRSProposedRDTaxAdjustment    
Excess tax benefit 0us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities 0us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities 0us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options 0us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions 0us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions  
Components of the provision for income taxes      
Federal 0us-gaap_CurrentFederalTaxExpenseBenefit 0us-gaap_CurrentFederalTaxExpenseBenefit 0us-gaap_CurrentFederalTaxExpenseBenefit
State 868,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit 2,121,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit 370,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Deferred portion of income tax expense (benefit) 397,780,000us-gaap_DeferredIncomeTaxExpenseBenefit 105,555,000us-gaap_DeferredIncomeTaxExpenseBenefit (29,638,000)us-gaap_DeferredIncomeTaxExpenseBenefit
Income tax (expense) benefit 398,648,000us-gaap_IncomeTaxExpenseBenefit 107,676,000us-gaap_IncomeTaxExpenseBenefit (29,268,000)us-gaap_IncomeTaxExpenseBenefit
Effective tax rate (as a percent) 37.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations 38.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Excess income tax benefit from the exercise of stock awards 0us-gaap_ExcessTaxBenefitFromShareBasedCompensationOperatingActivities    
Deferred Income Taxes [Abstract]      
Deferred Tax Liabilities, Oil and Gas Properties 1,029,424,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment 768,463,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment  
Deferred Tax Liabilities, Derivative asset 220,437,000us-gaap_DeferredTaxLiabilitiesDerivatives 9,529,000us-gaap_DeferredTaxLiabilitiesDerivatives  
Deferred Tax Liabilities, Other 4,475,000us-gaap_DeferredTaxLiabilitiesOther 1,245,000us-gaap_DeferredTaxLiabilitiesOther  
Deferred Tax Liabilities 1,254,336,000us-gaap_DeferredIncomeTaxLiabilities 779,237,000us-gaap_DeferredIncomeTaxLiabilities  
Deferred Tax Assets, Federal and state tax net operating loss carryovers 184,447,000us-gaap_OperatingLossCarryforwards 91,788,000us-gaap_OperatingLossCarryforwards  
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost 16,763,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost 18,820,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost  
Deferred Tax Assets, Stock compensation 126,700,000sm_UnrecognizedTaxBenefitsExerciseOfStockAwards    
Deferred Tax Assets, Other Long-term Liabilities 16,301,000sm_DeferredTaxAssetsOtherLongTermLiabilities 13,341,000sm_DeferredTaxAssetsOtherLongTermLiabilities  
Deferred Tax Assets, Net Profits Plan Liability 9,414,000sm_DeferredTaxAssetsNetProfitsPlanLiability 20,913,000sm_DeferredTaxAssetsNetProfitsPlanLiability  
Deferred Tax Assets, Gross 226,925,000us-gaap_DeferredTaxAssetsGross 144,862,000us-gaap_DeferredTaxAssetsGross  
Deferred Tax Assets, Valuation Allowance (7,246,000)us-gaap_DeferredTaxAssetsValuationAllowance (5,001,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Deferred Tax Assets, Net 219,679,000us-gaap_DeferredTaxAssetsNet 139,861,000us-gaap_DeferredTaxAssetsNet  
Deferred Tax Liabilities, Net 1,034,657,000us-gaap_DeferredTaxLiabilities 639,376,000us-gaap_DeferredTaxLiabilities  
Less: current deferred income tax liabilities (152,082,000)us-gaap_DeferredTaxLiabilitiesGrossCurrent (172,000)us-gaap_DeferredTaxLiabilitiesGrossCurrent  
Add: current deferred income tax assets 9,106,000us-gaap_DeferredTaxAssetsGrossCurrent 10,921,000us-gaap_DeferredTaxAssetsGrossCurrent  
Deferred income taxes; Net, non-current 891,681,000us-gaap_DeferredTaxLiabilitiesNoncurrent 650,125,000us-gaap_DeferredTaxLiabilitiesNoncurrent  
Deferred Tax Assets, Operating Loss Carryforwards, Domestic 626,200,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic    
Deferred Tax Assets, Operating Loss Carryforwards, State and Local 284,800,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal    
Deferred Tax Assets, Tax Credit Carryforwards, Research 3,500,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch    
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]      
Federal statutory tax expense (benefit) 372,644,000us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate 97,514,000us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate (29,231,000)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State tax expense (benefit) (net of federal benefit) 21,350,000us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 9,400,000us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes (992,000)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Research and development credit 0us-gaap_IncomeTaxReconciliationTaxCreditsResearch 0us-gaap_IncomeTaxReconciliationTaxCreditsResearch (970,000)us-gaap_IncomeTaxReconciliationTaxCreditsResearch
Change in valuation allowance 2,245,000us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance (314,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance 1,524,000us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Other 2,409,000us-gaap_IncomeTaxReconciliationOtherAdjustments 1,076,000us-gaap_IncomeTaxReconciliationOtherAdjustments 401,000us-gaap_IncomeTaxReconciliationOtherAdjustments
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized Tax Benefits, Beginning Balance 2,358,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 2,278,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 1,961,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Unrecognized Tax Benefits, Additions For Tax Positions Of Prior Years 140,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 80,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 317,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities (916,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
Unrecognized Tax Benefits, Ending Balance 1,582,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 2,358,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 2,278,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Internal Revenue Service (IRS) [Member]      
Deferred Income Taxes [Abstract]      
Income Taxes Receivable, Current 4,734,000us-gaap_IncomeTaxesReceivable
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		 4,630,000us-gaap_IncomeTaxesReceivable
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		  
State and Local Jurisdiction [Member]      
Deferred Income Taxes [Abstract]      
Income Taxes Receivable, Current 0us-gaap_IncomeTaxesReceivable
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		 0us-gaap_IncomeTaxesReceivable
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		  
Taxes Payable, Current 25,000us-gaap_TaxesPayableCurrent
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		 1,460,000us-gaap_TaxesPayableCurrent
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		  
Minimum [Member]      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized Tax Benefits, Ending Balance 0us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Maximum [Member]      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized Tax Benefits, Ending Balance $ 1,200,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember