COMMITMENTS AND CONTINGENCIES - Narrative (Details) - Meadowlark Midstream Gathering System	3 Months Ended
Mar. 31, 2021 USD ($)
Loss Contingencies [Line Items]
Legal fines and other fees	$ 2,500,000
Accrued loss contingency	17,000,000.0
Maximum \| Principal Owner
Loss Contingencies [Line Items]
Coverage from pollution liability insurance policy	25,000,000.0
Coverage from property and business interruption insurance policy	$ 200,000,000.0

Loss contingency, pollution liability insurance policy, coverage.

No definition available.

Name:	smlp_LossContingencyPollutionLiabilityInsurancePolicyCoverage
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loss contingency, property and business interruption insurance policy, coverage.

+ References

No definition available.

+ Details

Name:	smlp_LossContingencyPropertyAndBusinessInterruptionInsurancePolicyCoverage
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense for newly incurred environmental loss contingencies. Includes, but is not limited to, remediation expense and interest accretion.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 5.Y.Q2)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

+ Details

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

- Definition

Undiscounted amount of the accrual for environmental loss contingencies.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 5.Y.Q1)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

+ Details

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

- 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.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_EnvironmentalRemediationSiteAxis=smlp_MeadowlarkMidstreamGatheringSystemMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_PrincipalOwnerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: