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Compensation Plans Non Stock-based Compensation (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Document Period End Date Mar. 31, 2015  
Net Profits Plan [Member]    
Total Cash Payments, Made or Accrued under Profit Sharing Plan $ 1.3sm_TotalCashPaymentsMadeOrAccruedUnderProfitSharingPlan
/ us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis
= sm_NetProfitsPlanMember
$ 3.3sm_TotalCashPaymentsMadeOrAccruedUnderProfitSharingPlan
/ us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis
= sm_NetProfitsPlanMember