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Derivative Financial Instruments Credit Facility and Derivative Counterparties (Details) (Line of Credit [Member])	Dec. 31, 2014
Line of Credit [Member]
Line of Credit Facility [Line Items]
Percentage of Proved Oil and Gas Properties Secured for Credit Facility Borrowing	75.00%sm_PercentageofProvedOilandGasPropertiesSecuredforCreditFacilityBorrowing / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember

X

- Definition

Percentage of Proved Oil and Gas Properties Secured for Credit Facility Borrowing

+ References

No definition available.

+ Details

Name:	sm_PercentageofProvedOilandGasPropertiesSecuredforCreditFacilityBorrowing
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration