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Acquisition and Development Agreement and Carry and Earning Agreement (Details) (Mitsui E&P Texas LP, USD $) In Millions, unless otherwise specified | Jun. 30, 2011
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Mitsui E&P Texas LP |
Acquisition and Development Agreement [Abstract] |
Percentage of working interests transferred to acquiree entity (as a percent) | 12.50%sm_PercentageOfWorkingInterestsTransferredToAcquireeEntity / us-gaap_BusinessAcquisitionAxis = sm_MitsuiEAndPTexasLpMember
Acreage of working interests transferred to acquiree entity (in acres) | 39,000 sm_AcreageOfWorkingInterestsTransferredToAcquireeEntity / us-gaap_BusinessAcquisitionAxis = sm_MitsuiEAndPTexasLpMember
Percentage of the entity's costs and expenses during the first three years following the closing of the transaction, borne by the acquiree entity (as a percent) | 90.00%sm_PortionOfDrillingAndCompletionCostsCarriedByAcquirer / us-gaap_BusinessAcquisitionAxis = sm_MitsuiEAndPTexasLpMember
Costs and expenses incurred by acquiree entity on behalf of entity that the acquiree entity has agreed to pay | $ 680.0 sm_CostsAndExpensesIncurredByAcquireeEntityOnBehalfOfEntityThatAcquireeEntityAgreedToPay / us-gaap_BusinessAcquisitionAxis = sm_MitsuiEAndPTexasLpMember
Portion of Drilling and Completion Costs Not Carried by Acquirer | 10.00%sm_PortionOfDrillingAndCompletionCostsNotCarriedByAcquirer / us-gaap_BusinessAcquisitionAxis = sm_MitsuiEAndPTexasLpMember

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- Details
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Namespace Prefix: | sm_
Data Type: | xbrli:stringItemType
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- Definition Represents the acreage of working interests transferred to acquiree entity. + References No definition available. + Details
Name: | sm_AcreageOfWorkingInterestsTransferredToAcquireeEntity
Namespace Prefix: | sm_
Data Type: | xbrli:decimalItemType
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- Definition Represents the costs and expenses incurred by acquiree entity on behalf of entity which the acquiree entity would repay on a proportionate basis. + References No definition available. + Details
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- Definition Represents the percentage of working interests transferred to acquiree entity. + References No definition available. + Details
Name: | sm_PercentageOfWorkingInterestsTransferredToAcquireeEntity
Namespace Prefix: | sm_
Data Type: | num:percentItemType
Balance Type: | na
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- Definition Represents the percentage of costs incurred by the entity, which the acquiree entity has agreed to bear for first three years following the closing of transaction. + References No definition available. + Details
Name: | sm_PortionOfDrillingAndCompletionCostsCarriedByAcquirer
Namespace Prefix: | sm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Portion of Drilling and Completion Costs Not Carried by Acquirer + References No definition available. + Details
Name: | sm_PortionOfDrillingAndCompletionCostsNotCarriedByAcquirer
Namespace Prefix: | sm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant