Fair Value Measurements (Details 2) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Proved Oil and Gas Properties
Discount rate used for fair value of oil and gas properties (as a percent)	12.00%		12.00%
Period of New York Mercantile Exchange ("NYMEX") strip pricing used for price forecast (in years)			5
Net Profits Plan liability
Liability measured at fair value using Level 3 inputs
Discount rate to calculate liability pools currently in payout (as a percent)	12.00%		12.00%
Discount rate to calculate liability pools not currently in payout (as a percent)	15.00%		15.00%
Number of periods used for price assumptions of strip prices of liabilities			5
Period used for price assumptions of strip prices of liabilities (in years)			1
Percentage change in commodity prices (as a percent)			5.00%
Change in liability due to change in commodity prices by 5 percent	$ 10,000,000		$ 10,000,000
Percentage increase in discount rate (as a percent)			1.00%
Percentage decrease in discount rate (as a percent)			1.00%
Increase in liability due to a 1 percent decrease in discount rate	7,000,000		7,000,000
Decrease in liability due to a 1 percent increase in discount rate	6,000,000		6,000,000
Liability measured at fair value using Level 3 inputs
Beginning balance	147,403,000	143,019,000	135,850,000	170,291,000
Net increase (decrease) in liability	(6,092,000)	1,318,000	18,193,000	(218,000)
Net settlements	(7,892,000)	(7,917,000)	(20,624,000)	(33,653,000)
Ending balance	133,419,000	136,420,000	133,419,000	136,420,000
Amount of cash payments related to divested properties	2,000,000	1,900,000	6,300,000	20,100,000
Amount paid for settlement of liabilities	$ 2,600,000

Cash payments made or accrued related to profit sharing plan liabilities arising from divested properties.

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Balance Type:	debit
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- Definition

Rate applied to the undiscounted amount of estimated cash flows from proved oil and gas properties to arrive at the present value recorded as of the balance sheet date.

+ Details

Name:	sm_DiscountRateUsedForFairValueOfOilAndGasProperties
Namespace Prefix:	sm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage used to calculate the estimated present value of expected future payment of liability related to deferred compensation plan that have reached payout.

+ Details

Name:	sm_DiscountRateUsedToCalculateCurrentlyInPayoutLiabilities
Namespace Prefix:	sm
Data Type:	num:percentItemType
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Period Type:	instant

- Definition

Represents the percentage used to calculate the estimated present value of expected future payment of liability related to deferred compensation plan that have not reached payout.

+ Details

Name:	sm_DiscountRateUsedToCalculateNotCurrentlyInPayoutLiabilities
Namespace Prefix:	sm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of periods used for price assumptions of strip prices of liabilities.

+ Details

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Namespace Prefix:	sm
Data Type:	xbrli:integerItemType
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Period Type:	duration

- Details

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Balance Type:	na
Period Type:	duration

- Definition

The amount of cash used to directly settle a portion of the net profit plan liability.

+ Details

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Namespace Prefix:	sm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Change in commodity prices expressed in percent for sensitivity analysis.

+ Details

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Namespace Prefix:	sm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Decrease in commodity prices expressed in percent for sensitivity analysis.

+ Details

Name:	sm_PercentDecreaseInDiscountRateForSensitivityAnalysis
Namespace Prefix:	sm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase in commodity prices expressed in percent for sensitivity analysis.

+ Details

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Namespace Prefix:	sm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period of New York Mercantile Exchange (NYMEX) strip pricing used for price forecast.

+ Details

Name:	sm_PeriodOfNewYorkMercantileExchangeStripPricingUsedForPriceForecast
Namespace Prefix:	sm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period used for price assumptions of strip prices of liabilities.

+ Details

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Namespace Prefix:	sm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Change in the financial asset or liability resulting from sensitivity analysis due to change in commodity prices.

+ Details

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Namespace Prefix:	sm
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- Definition

Change in the financial asset or liability resulting from sensitivity analysis due to decrease in the discount rate.

+ Details

Name:	sm_SensitivityAnalysisDecreaseInLiabilityDueToChangeInDiscountRate
Namespace Prefix:	sm
Data Type:	xbrli:monetaryItemType
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- Definition

Change in the financial asset or liability resulting from sensitivity analysis due to increase in the discount rate.

+ Details

Name:	sm_SensitivityAnalysisIncreaseInLiabilityDueToChangeInDiscountRate
Namespace Prefix:	sm
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

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- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 157
-Paragraph 32
-Subparagraph c(1)
-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 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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- Definition

Settlements that have taken place during the period in relation to liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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- Definition

This element represents a liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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