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Pension Benefits (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Defined Benefit Plan, Benefit Obligation Beginning of Year $ 43,285,000us-gaap_DefinedBenefitPlanBenefitObligation $ 40,237,000us-gaap_DefinedBenefitPlanBenefitObligation  
Defined Benefit Plan, Service Cost 6,335,000us-gaap_DefinedBenefitPlanServiceCost 6,291,000us-gaap_DefinedBenefitPlanServiceCost 4,934,000us-gaap_DefinedBenefitPlanServiceCost
Defined Benefit Plan, Interest Cost 2,191,000us-gaap_DefinedBenefitPlanInterestCost 1,627,000us-gaap_DefinedBenefitPlanInterestCost 1,374,000us-gaap_DefinedBenefitPlanInterestCost
Defined Benefit Plan, Amendments 0us-gaap_DefinedBenefitPlanPlanAmendments 0us-gaap_DefinedBenefitPlanPlanAmendments  
Defined Benefit Plan, Actuarial Gain (Loss) 8,821,000us-gaap_DefinedBenefitPlanActuarialGainLoss (1,577,000)us-gaap_DefinedBenefitPlanActuarialGainLoss  
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Actuarial Gain (Loss), before Tax (10,062,000)us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax 2,766,000us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax (4,680,000)us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeLossNetGainLossBeforeTax
Defined Benefit Plan, Benefits Paid (2,765,000)us-gaap_DefinedBenefitPlanBenefitsPaid (3,293,000)us-gaap_DefinedBenefitPlanBenefitsPaid (1,000,000)us-gaap_DefinedBenefitPlanBenefitsPaid
Defined Benefit Plan, Benefit Obligation End of Year 57,867,000us-gaap_DefinedBenefitPlanBenefitObligation 43,285,000us-gaap_DefinedBenefitPlanBenefitObligation 40,237,000us-gaap_DefinedBenefitPlanBenefitObligation
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year 24,658,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 20,254,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets  
Defined Benefit Plan, Actual Return on Plan Assets 737,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets 2,726,000us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets  
Defined Benefit Plan, Employer Contribution 5,310,000us-gaap_DefinedBenefitPlanContributionsByEmployer 4,971,000us-gaap_DefinedBenefitPlanContributionsByEmployer 5,400,000us-gaap_DefinedBenefitPlanContributionsByEmployer
Defined Benefit Plan, Fair Value of Plan Assets End of Year 27,940,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 24,658,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 20,254,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Defined Benefit Plan, Funded Status of Plan (29,927,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan (18,627,000)us-gaap_DefinedBenefitPlanFundedStatusOfPlan  
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]      
Defined Benefit Plan, Benefit Obligation End of Year 57,867,000us-gaap_DefinedBenefitPlanBenefitObligation 43,285,000us-gaap_DefinedBenefitPlanBenefitObligation 40,237,000us-gaap_DefinedBenefitPlanBenefitObligation
Defined Benefit Plan, Accumulated Benefit Obligation 43,205,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation 32,396,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation  
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year (27,940,000)us-gaap_DefinedBenefitPlanFairValueOfPlanAssets (24,658,000)us-gaap_DefinedBenefitPlanFairValueOfPlanAssets (20,254,000)us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Defined Benefit Plan, Accumulated Unfunded Benefit Obligation 15,265,000sm_DefinedBenefitPlanAccumulatedUnfundedBenefitObligation 7,738,000sm_DefinedBenefitPlanAccumulatedUnfundedBenefitObligation  
Pre-tax Amounts Recognized in Accumulated Other Comprehensive Income      
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax 17,812,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax 8,439,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax  
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax 118,000us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax 136,000us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax  
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Transition Assets (Obligations), before Tax 0us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetTransitionAssetsObligationsBeforeTax 0us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetTransitionAssetsObligationsBeforeTax  
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax 17,930,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax 8,575,000us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax  
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]      
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Arising During Period, before Tax 0us-gaap_OtherComprehensiveIncomeDefinedBenefitPlanNetPriorServiceCostCreditArisingDuringPeriodBeforeTax 0us-gaap_OtherComprehensiveIncomeDefinedBenefitPlanNetPriorServiceCostCreditArisingDuringPeriodBeforeTax 0us-gaap_OtherComprehensiveIncomeDefinedBenefitPlanNetPriorServiceCostCreditArisingDuringPeriodBeforeTax
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Recognized in Net Periodic Benefit Cost, before Tax (17,000)us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax (17,000)us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax (17,000)us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditBeforeTax
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net (Gain) Loss Recognized in Net Periodic Benefit Cost, Net of Tax (689,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax (1,222,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax (754,000)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax
Total Other Comprehensive Income (Loss) (9,356,000)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax 4,005,000us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax (3,909,000)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax
Components of Net Periodic Benefit Costs for Both Pension Plans      
Defined Benefit Plan, Service Cost 6,335,000us-gaap_DefinedBenefitPlanServiceCost 6,291,000us-gaap_DefinedBenefitPlanServiceCost 4,934,000us-gaap_DefinedBenefitPlanServiceCost
Defined Benefit Plan, Interest Cost 2,191,000us-gaap_DefinedBenefitPlanInterestCost 1,627,000us-gaap_DefinedBenefitPlanInterestCost 1,374,000us-gaap_DefinedBenefitPlanInterestCost
Defined Benefit Plan, Expected Return on Plan Assets (1,978,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (1,538,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets (1,165,000)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 17,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 17,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit 17,000us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
Defined Benefit Plan, Amortization of Gains (Losses) 689,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 1,222,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses 754,000us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
Defined Benefit Plan, Net Periodic Benefit Cost 7,254,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 7,619,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost 5,914,000us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation and Net Periodic Benefit Cost [Abstract]      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.30%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate 3.90%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase 6.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease 6.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease 6.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 5.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 3.90%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate 4.70%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets [1] 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets [1] 7.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase 6.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 6.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease 6.20%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities 100.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations    
Defined Benefit Plan, Asset Allocation 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations  
Future Benefit Payments      
Defined Benefit Plan, Expected Future Benefit Payments in Year One 3,006,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths    
Defined Benefit Plan, Expected Future Benefit Payments in Year Two 3,046,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo    
Defined Benefit Plan, Expected Future Benefit Payments in Year Three 3,921,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree    
Defined Benefit Plan, Expected Future Benefit Payments in Year Four 4,558,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour    
Defined Benefit Plan, Expected Future Benefit Payments in Year Five 5,204,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive    
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter 38,427,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter    
Pension And Other Post-retirement Benefit Plans, Gain (Loss) Amortization Threshold 10.00%sm_PensionAndOtherPostRetirementBenefitPlansGainLossAmortizationThreshold    
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year 1,200,000us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear    
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year 5,800,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear    
Equity Securities [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year 10,744,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		    
Defined Benefit Plan, Fair Value of Plan Assets End of Year 11,067,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		    
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]      
Defined Benefit Plan, Fair Value of Plan Assets Beginning of Year $ (11,067,000)us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		    
Defined Benefit Plan, Assets, Target Allocations [Abstract]      
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities 42.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		    
Defined Benefit Plan, Asset Allocation 39.60%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 43.60%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		  
[1] (1)There is no assumed expected return on plan assets for the Nonqualified Pension Plan because there are no plan assets in the Nonqualified Pension Plan.