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Income Taxes Narrative (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2015 USD ($)
Income Tax Narrative [Abstract] |
Tax Credit Recorded for IRS R&D Settlement | $ 2.0

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- Definition Income Tax Narrative [Abstract] + References No definition available. + Details
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Namespace Prefix: | sm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Tax Credit Recorded for IRS R&D Settlement + References No definition available. + Details
Name: | sm_TaxCreditRecordedforIRSRDSettlement
Namespace Prefix: | sm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration