v2.4.0.6

Long-Term Debt (Details) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
6.50% Senior Notes [Member]
Long-term debt
Debt Instrument, Face Amount	$ 350.0
Interest rate on notes (as a percent)	6.50%	6.50%
6.625% Senior Notes [Member]
Long-term debt
Debt Instrument, Face Amount	350.0
Interest rate on notes (as a percent)	6.625%	6.625%
Convertible Debt [Member]
Long-term debt
Interest rate on notes (as a percent)	3.50%	3.50%
Stock Issued During Period, Shares, Conversion of Convertible Securities	864,106
cash paid for redemption of convertible debt	6.2
Repayments of Long-term Debt	281.3
6.50% Senior Notes due 2023 [Member]
Long-term debt
Debt Instrument, Face Amount	400.0
Interest rate on notes (as a percent)	6.50%
Proceeds from Debt, Net of Issuance Costs	392.3
Deferred Finance Costs, Gross	7.7
Debt Instrument Future Redemption Amount As Percentage Of Principal	35.00%
Debt Instrument Future Redemption Price As Percentageof Principal First Through Third Year	106.50%
Senior Notes due 2023 Redemption Schedule [Abstract]
Debt Instrument, Future Redemption Price as Percentage of Principal, Year 5	103.25%
Debt Instrument Future Redemption Price as Percentage of Principal, Year 6	102.167%
Debt Instrument, Future Redemption Price as Percentage of Principal, Year 7	101.083%
Debt Instrument, Future Redemption Price as Percentage of Principal, Year 8 and Thereafter	100.00%
Debt Instrument, Covenant Compliance, Dividends Excluded From Computation	$ 6.5
Debt Instrument, Additional Interest Rate Maximum for Failure to File Exchange Offer or Ineffective Shelf Registration Statement	1.00%
Debt Instrument Future Redemption Price As Percentage Of Principal, First Five Year	100.00%

X

- Definition

Cash used to extingush convertible debt

+ References

No definition available.

+ Details

Name:	sm_CashPaidForConversionOfConvertibleSecurities
Namespace Prefix:	sm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Cash Paid for Redemption of Convertible Debt

+ References

No definition available.

+ Details

Name:	sm_CashPaidForRedemptionOfConvertibleDebt
Namespace Prefix:	sm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Represents the maximum additional interest rate to be added to the debt instrument if the entity fails to complete the exchange offer of the shelf registration statement within the prescribed time.

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentAdditionalInterestRateMaximumForFailureToFileExchangeOfferOrIneffectiveShelfRegistrationStatement
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the amount of dividend payments excluded from the resticted dividend calculation covenant.

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentCovenantComplianceDividendsExcludedFromComputation
Namespace Prefix:	sm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Represents the maximum contractual redemption amount as a percentage of the original principle amount, plus accrued interest, within the first three years of issuance.

+ References

No definition available.

+ Details

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

X

- Definition

Debt Instrument Future Redemption Price As Percentage Of Principal, First Five Years

+ References

No definition available.

+ Details

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

X

- Definition

Represents the redemption price of the debt instrument as a percentage of principal, plus accrued interest, for the first three years of issuance.

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentFutureRedemptionPriceAsPercentageofPrincipalFirstThroughThirdYear
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Debt Instrument, Future Redemption Price as Percentage of Principal, Year 5

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentFutureRedemptionPriceAsPercentageOfPrincipalYear5
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Debt Instrument, Future Redemption Price as Percentage of Principal, Year 6

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentFutureRedemptionPriceAsPercentageOfPrincipalYear6
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Debt Instrument, Future Redemption Price as Percentage of Principal, Year 7

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentFutureRedemptionPriceAsPercentageOfPrincipalYear7
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Debt Instrument, Future Redemption Price as Percentage of Principal, Year 8 and Thereafter

+ References

No definition available.

+ Details

Name:	sm_DebtInstrumentFutureRedemptionPriceAsPercentageOfPrincipalYear8AndThereafter
Namespace Prefix:	sm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	sm_SeniorNotesDue2023RedemptionScheduleAbstract
Namespace Prefix:	sm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

+ References

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-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.



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Interest rate stated in the contractual debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-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 210.5-02.22(a)(1))

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

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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

X

- Definition

For an unclassified balance sheet, the amount as of the balance sheet date of capitalized costs associated with the issuance of debt instruments (for example, legal, accounting, underwriting, printing, and registration costs) that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization.

+ References

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16

-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.



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399

+ Details

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

X

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

No definition available.

+ Details

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

X

- Definition

Number of shares issued during the period as a result of the conversion of convertible securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 4, 5

-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.



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 12

-Paragraph 10

-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.



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29-30)

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



Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=6959260&loc=d3e187085-122770



Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5



Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration