Suspended Well Costs (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized Exploratory Well Costs Beginning Balance	$ 34,527us-gaap_CapitalizedExploratoryWellCosts	$ 9,100us-gaap_CapitalizedExploratoryWellCosts	$ 18,600us-gaap_CapitalizedExploratoryWellCosts
Capitalized Exploratory Well Cost, Additions Pending Determination of Proved Reserves	43,589	34,527	9,100
Reclassification to Well, Facilities, and Equipment Based on Determination of Proved Reserves	(33,340)	(9,100)	(5,865)
Capitalized Exploratory Well Cost, Charged to Expense	(1,187)	0	(12,735)
Capitalized Exploratory Well Costs Ending Balance	43,589us-gaap_CapitalizedExploratoryWellCosts	34,527us-gaap_CapitalizedExploratoryWellCosts	9,100us-gaap_CapitalizedExploratoryWellCosts
Exploratory Well Costs Capitalized Greater than One Year	$ 0

Exploratory Well Costs Capitalized Greater than One Year

No definition available.

Name:	sm_ExploratoryWellCostsCapitalizedGreaterthanOneYear
Namespace Prefix:	sm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Additions to capitalized exploratory well costs that were added during the reporting period pending the determination of proved reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=SL6806780-109447

+ Details

Name:	us-gaap_CapitalizedExploratoryWellCostAdditionsPendingDeterminationOfProvedReserves
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Capitalized exploratory well costs that were expensed during the year because proved reserves were not found at the exploratory well.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=SL6806780-109447

+ Details

Name:	us-gaap_CapitalizedExploratoryWellCostChargedToExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of capitalized exploratory well costs that is pending the determination of proved reserves. This does not include amounts that were capitalized and subsequently expensed in the same annual period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=SL6806780-109447

+ Details

Name:	us-gaap_CapitalizedExploratoryWellCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Previously capitalized costs of the drilling of wells that were reclassified as part of the costs of the enterprise's wells and related equipment and facilities at the time that proved reserves were found.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=SL6806780-109447

+ Details

Name:	us-gaap_ReclassificationToWellFacilitiesAndEquipmentBasedOnDeterminationOfProvedReserves
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration