Assumption used in valuing an instrument.

No definition available.

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

- Definition

Impairment of Proved, Unproved, and Other Properties [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Major Customers [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Number of individual entities within groups of entities under common ownership that represent more than 10 percent of total production revenue

+ References

No definition available.

+ Details

Name:	sm_Numberofindividualentitieswithingroupsofentitiesundercommonownershipthatrepresentmorethan10percentoftotalproductionrevenue
Namespace Prefix:	sm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	sm_PeriodofNewYorkMercantileExchangeStripPricingUsedforPriceForecast
Namespace Prefix:	sm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue Receipt, Days After Sale

+ References

No definition available.

+ Details

Name:	sm_RevenueReceiptDaysAfterSale
Namespace Prefix:	sm_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_OilAndGasPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=sm_OilGasandNGLsSalesRevenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_MajorCustomerOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_MajorCustomerTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_MajorCustomerThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_MajorCustomerFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_MajorCustomerFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_UnnamedMajorCustomerGroupOnewithRelatedEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=sm_UnnamedMajorCustomerGroupTwowithRelatedEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_PropertyPlantAndEquipmentOtherTypesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputRiskFreeInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=us-gaap_AssetRetirementObligationCostsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Summary of Significant Accounting Policies Other (Details)		12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		100.00%	100.00%	100.00%
Number of individual entities within groups of entities under common ownership that represent more than 10 percent of total production revenue		0
Line of Credit [Member]
Long-term Debt, Unclassified [Abstract]
Revolving credit facility	$ 93,000,000.0	$ 93,000,000.0	$ 122,500,000
Oil and Gas Properties [Member]
Impairment of Proved, Unproved, and Other Properties [Abstract]
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast	5 years
Measurement Input, Discount Rate [Member] \| Fair Value, Nonrecurring [Member] \| Oil and Gas Properties [Member]
Impairment of Proved, Unproved, and Other Properties [Abstract]
Fair value assumptions, measurement input	0.11	0.11
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Major customer one [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		0.00%	18.00%	18.00%
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Major customer two [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		20.00%	14.00%	5.00%
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Major customer three [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		24.00%	13.00%	7.00%
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Major customer four [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		6.00%	9.00%	10.00%
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Major customer five [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		15.00%	4.00%	0.00%
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Group one of entities under common control [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		5.00%	13.00%	18.00%
Customer Concentration Risk [Member] \| Oil, gas, and NGL revenue [Member] \| Group two of entities under common control [Member]
Concentration of Credit Risk and Major Customers [Abstract]
Entity-Wide Revenue, Major Customer, Percentage		7.00%	11.00%	12.00%
Minimum [Member]
Revenues [Abstract]
Revenue receipt, days after sale		30
Minimum [Member] \| Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment, Estimated Useful Lives		3 years
Minimum [Member] \| Measurement Input, Discount Rate [Member] \| Fair Value, Nonrecurring [Member] \| Oil and Gas Properties [Member]
Impairment of Proved, Unproved, and Other Properties [Abstract]
Fair value assumptions, measurement input	0.10	0.10
Minimum [Member] \| Measurement Input, Credit adjusted risk free rate [Member] \| Asset Retirement Obligation Costs [Member]
Impairment of Proved, Unproved, and Other Properties [Abstract]
Fair value assumptions, measurement input	0.055	0.055
Maximum [Member]
Revenues [Abstract]
Revenue receipt, days after sale		90
Maximum [Member] \| Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment, Estimated Useful Lives		30 years
Maximum [Member] \| Measurement Input, Discount Rate [Member] \| Fair Value, Nonrecurring [Member] \| Oil and Gas Properties [Member]
Impairment of Proved, Unproved, and Other Properties [Abstract]
Fair value assumptions, measurement input	0.15	0.15
Maximum [Member] \| Measurement Input, Credit adjusted risk free rate [Member] \| Asset Retirement Obligation Costs [Member]
Impairment of Proved, Unproved, and Other Properties [Abstract]
Fair value assumptions, measurement input	0.12	0.12