Represents the acreage of working interests transferred to acquiree entity.

No definition available.

Name:	sm_AcreageOfWorkingInterestsTransferredToAcquireeEntity
Namespace Prefix:	sm_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- 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

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

- 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_CostsIncurredByEntityPercentageBorneByAcquireeEntityForFirstThreeYearsFollowingCloseOfTransaction
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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
Period Type:	instant

- Definition

Percent of original share in drilling and completion costs retained after agreement with 3rd party was reached

+ References

No definition available.

+ Details

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

- Definition

Total amount used at the balance sheet date of the Mitsui Acquisition and Development Agreement

+ References

No definition available.

+ Details

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

- Details

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

- Definition

Cash and equivalents whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 1

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.1)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

Acquisition and Development Agreement (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Mitsui E&P Texas LP	Jun. 30, 2011 Mitsui E&P Texas LP
Acquisition and development agreement
Percentage of working interests transferred to acquiree entity (as a percent)				12.50%
Acreage of working interests transferred to acquiree entity (in acres)				39,000
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%
Costs and expenses incurred by acquiree entity on behalf of entity that the acquiree entity has agreed to pay				$ 680,000,000
RetainedShareOfDrillingAndCompletionCosts	10.00%
Restricted Cash and Cash Equivalents, Noncurrent	109,486,000	124,703,000
Utilization Amount of Carry and Earning Agreement			$ 128,300,000