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Compensation Plans Non Stock-based Compensation (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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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 | $ 1.3 sm_TotalCashPaymentsMadeOrAccruedUnderProfitSharingPlan / us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis = sm_NetProfitsPlanMember | $ 3.3 sm_TotalCashPaymentsMadeOrAccruedUnderProfitSharingPlan / us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis = sm_NetProfitsPlanMember

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- Definition The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Total amount expensed related to cash payments made or accrued under the profit sharing plan. + References No definition available. + Details
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Namespace Prefix: | sm_
Data Type: | xbrli:monetaryItemType
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |