AMORTIZING INTANGIBLE ASSETS AND UNFAVORABLE GAS GATHERING CONTRACT - Estimated Aggregate Annual Amortization Expected to be Recognized (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Intangible assets
2018	$ 17,053
2019	33,217
2020	33,041
2021	29,349
2022	26,282
Unfavorable gas gathering contract
2018	809
2019	0
2020	0
2021	0
2022	$ 0

Finite lived noncurrent liability future amortization income.

No definition available.

Name:	smlp_FiniteLivedNoncurrentLiabilityFutureAmortizationIncomeAbstract
Namespace Prefix:	smlp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Finite lived unfavorable contract amortization income, remainder of fiscal year.

+ References

No definition available.

+ Details

Name:	smlp_FiniteLivedUnfavorableContractAmortizationIncomeRemainderOfFiscalYear
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income expected to be recognized during the fifth fiscal year following the latest fiscal year for unfavorable contract with a finite life.

+ References

No definition available.

+ Details

Name:	smlp_FiniteLivedUnfavorableContractAmortizationIncomeYearFive
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income expected to be recognized during the fourth fiscal year following the latest fiscal year for unfavorable contract with a finite life.

+ References

No definition available.

+ Details

Name:	smlp_FiniteLivedUnfavorableContractAmortizationIncomeYearFour
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income expected to be recognized during the third fiscal year following the latest fiscal year for unfavorable contract with a finite life.

+ References

No definition available.

+ Details

Name:	smlp_FiniteLivedUnfavorableContractAmortizationIncomeYearThree
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization income expected to be recognized during the second fiscal year following the latest fiscal year for unfavorable contract with a finite life.

+ References

No definition available.

+ Details

Name:	smlp_FiniteLivedUnfavorableContractAmortizationIncomeYearTwo
Namespace Prefix:	smlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized in the remainder of the fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

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

- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- References

No definition available.

+ Details

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