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Assets Held for Sale (Details) (USD $) In Thousands, unless otherwise specified | 6 Months Ended |
--- | --- | ---
Jun. 30, 2013 | Dec. 31, 2012
Assets held for sale [Abstract] |  |
Assets Held-for-sale Reasonably Certain Period for Sale | 1 |
Asset retirement obligation associated with oil and gas properties held for sale | $ 4,617 | $ 1,393
Oil and gas properties held for sale net of accumulated depletion, depreciation and amortization of $55,348 in 2013 and 20,676 in 2012 | $ 87,310 | $ 33,620

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- Definition Noncurrent portion of the carrying amount of a liability for an asset retirement obligation for an asset held for sale. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References No definition available. + Details
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Namespace Prefix: | sm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | sm_AssetsheldforsaleAbstract
Namespace Prefix: | sm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the period where there is reasonable certainty that the sale will take place for the assets to be classified as held for sale. + References No definition available. + Details
Name: | sm_AssetsHeldForSaleReasonablyCertainPeriodForSale
Namespace Prefix: | sm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Long-lived assets that are held for sale apart from normal operations and anticipated to be sold in less than one year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1107-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 144 -Paragraph 46 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant