Summit Midstream Partners (SMLP) 10-K2 Mar 152014 FY Annual reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
Document and Entity Information (USD $)	Dec. 31, 2014	Jun. 30, 2014	Jan. 31, 2015
Entity Information [Line Items]
Entity Registrant Name	Summit Midstream Partners, LP
Entity Central Index Key	1549922
Document Type	10-K
Document Period End Date	31-Dec-14
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float		$1,260,163,276
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Common units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			34,426,513
Subordinated Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			24,409,850
General Partner Units
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			1,200,651

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$26,428	$20,357
Accounts receivable	83,612	67,877
Other current assets	3,289	4,741
Total current assets	113,329	92,975
Property, plant and equipment, net	1,235,652	1,158,081
Intangible assets, net	466,866	502,177
Goodwill	61,689	115,888
Other noncurrent assets	17,338	14,618
Total assets	1,894,874	1,883,739
Current liabilities:
Trade accounts payable	12,852	25,117
Due to affiliate	2,711	653
Deferred revenue	2,377	1,555
Ad valorem taxes payable	8,717	8,375
Accrued interest	18,858	12,144
Other current liabilities	11,939	11,729
Total current liabilities	57,454	59,573
Long-term debt	808,000	586,000
Noncurrent liability, net	5,577	6,374
Deferred revenue	55,239	29,683
Other noncurrent liabilities	1,715	372
Total liabilities	927,985	682,002
Commitments and contingencies
Common limited partner capital (34,427 units issued and outstanding at December 31, 2014 and 29,080 units issued and outstanding at December 31, 2013)	649,060	566,532
Subordinated limited partner capital (24,410 units issued and outstanding at December 31, 2014 and 2013)	293,153	379,287
General partner interests (1,201 units issued and outstanding at December 31, 2014 and 1,091 units issued and outstanding at December 31, 2013)	24,676	23,324
Summit Investments' equity in contributed subsidiaries	0	232,594
Total partners' capital	966,889	1,201,737
Total liabilities and partners' capital	$1,894,874	$1,883,739

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Common limited partner capital, units issued	34,427	29,080
Common limited partner capital, units outstanding	34,427	29,080
Subordinated limited partner capital, units issued	24,410	24,410
Subordinated limited partner capital, units outstanding	24,410	24,410
General partner interests, units issued	1,201	1,091
General partner interests, units outstanding	1,201	1,091

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS (USD $)	12 Months Ended
Share data in Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Gathering services and other fees	$235,033,000	$205,346,000	$154,139,000
Natural gas, NGLs and condensate sales and other	96,597,000	88,606,000	20,476,000
Amortization of favorable and unfavorable contracts	-944,000	-1,032,000	-192,000
Total revenues	330,686,000	292,920,000	174,423,000
Costs and expenses:
Cost of natural gas and NGLs	58,094,000	44,233,000	3,224,000
Operation and maintenance	76,272,000	72,465,000	53,882,000
General and administrative	34,017,000	30,105,000	22,182,000
Transaction costs	730,000	2,841,000	2,025,000
Depreciation and amortization	82,990,000	69,962,000	36,674,000
Loss on asset sales, net	442,000	113,000	0
Goodwill impairment	54,199,000	0	0
Long-lived asset impairment	5,505,000	0	0
Total costs and expenses	312,249,000	219,719,000	117,987,000
Other income	1,189,000	5,000	9,000
Interest expense	-40,159,000	-19,173,000	-7,340,000
Affiliated interest expense	0	0	-5,426,000
(Loss) Income before income taxes	-20,533,000	54,033,000	43,679,000
Income tax expense	-631,000	-729,000	-682,000
Net (loss) income	-21,164,000	53,304,000	42,997,000
Less: net income attributable to the pre-IPO period	0	0	24,112,000
Less: net income attributable to Summit Investments	2,828,000	9,720,000	1,271,000
Net (loss) income attributable to SMLP	-23,992,000	43,584,000	17,614,000
Less: net income attributable to general partner, including IDRs	3,125,000	1,035,000	352,000
Net (loss) income attributable to limited partners	-27,117,000	42,549,000	17,262,000
Weighted-average limited partner units outstanding
Cash distributions declared per common unit (in dollars per unit)	$2.04	$1.73
Common units
Costs and expenses:
Net (loss) income attributable to limited partners	-16,324,000	23,227,000	8,632,000
(Loss) earnings per limited partner unit
Basic (in dollars per share)	($0.49)	$0.86	$0.35
Diluted (in dollars per share)	($0.49)	$0.86	$0.35
Weighted-average limited partner units outstanding
Basic (shares)	33,311	26,951	24,412
Diluted (shares)	33,311	27,101	24,544
Subordinated Units
Costs and expenses:
Net (loss) income attributable to limited partners	($10,793,000)	$19,322,000	$8,630,000
(Loss) earnings per limited partner unit
Basic (in dollars per share)	($0.44)	$0.79	$0.35
Diluted (in dollars per share)	($0.44)	$0.79	$0.35
Weighted-average limited partner units outstanding
Basic (shares)	24,410	24,410	24,410
Diluted (shares)	24,410	24,410	24,410

CONSOLIDATED_STATEMENTS_OF_PAR

CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL AND MEMBERSHIP INTERESTS (USD $)	Total	SMLP units	Class B Units	Summit Investments' equity in contributed subsidiaries	Summit Investments' equity in contributed subsidiaries	Membership Interests	Membership Interests	Limited partners, Common	Limited partners, Common	Limited partners, Common	Limited partners, Subordinated	General partner	Bison Midstream	Bison Midstream	Bison Midstream	Bison Midstream	Bison Midstream	Mountaineer Midstream	Mountaineer Midstream	Mountaineer Midstream
In Thousands, unless otherwise specified	Total	SMLP units	Class B Units	Summit Investments' equity in contributed subsidiaries	Class B Units	Membership Interests	Class B Units	Limited partners, Common	SMLP units	Class B Units	Limited partners, Subordinated	General partner	Bison Midstream	Summit Investments' equity in contributed subsidiaries	Limited partners, Common	Limited partners, Subordinated	General partner	Mountaineer Midstream	Limited partners, Common	General partner
Beginning balance at Dec. 31, 2011	$640,818			$0		$640,818		$0			$0	$0
Increase (Decrease) in Partners' Capital [Roll Forward]
Net (loss) income	42,997			1,271		24,112		8,631			8,631	352
Unit-based compensation		269	1,607				1,793		269	-186
Net assets retained by the Predecessor	-4,417					-4,417
Contribution from general partner						-662,306		211,938			430,498	19,870
Assets contributed to Red Rock Gathering from Summit Investments	0
Issuance of common units, net of offering costs	262,382							262,382
Distributions	-123,138							-64,178			-58,960
Consolidation of net assets	206,694			206,694
Contribution of net assets from Summit Investments in excess of consideration paid for Bison Midstream	0
Cash advance from Summit Investments to contributed subsidiaries, net	500			500
Capitalized interest allocated to Red Rock Gathering projects from Summit Investments	0
Expenses paid by Summit Investments on behalf of contributed subsidiaries	2,536			2,536
Capital expenditures paid by Summit Investments on behalf of Red Rock Gathering	0
Ending balance at Dec. 31, 2012	1,030,248			211,001		0		418,856			380,169	20,222
Increase (Decrease) in Partners' Capital [Roll Forward]
Net (loss) income	53,304			9,720				22,311			20,238	1,035
Unit-based compensation		2,999	507		490				2,999	17
Net assets retained by the Predecessor	0
Assets contributed to Red Rock Gathering from Summit Investments	0
Distributions	-90,196							-46,286			-42,107	-1,803
Consolidation of net assets	303,168			303,168
Contribution from Summit Investments to Bison Midstream													2,229	2,229
Units issued to acquire entity													-200,000	-248,914	47,936		978	100,000	98,000	2,000
Contribution of net assets from Summit Investments in excess of consideration paid for Bison Midstream	56,535												0	-56,535	28,558	26,846	1,131
Repurchase of DFW Net Profits Interests	-11,957							-5,859			-5,859	-239
Cash advance from Summit Investments to contributed subsidiaries, net	738			738
Capitalized interest allocated to Red Rock Gathering projects from Summit Investments	496			496
Expenses paid by Summit Investments on behalf of contributed subsidiaries	10,149			10,149
Capital expenditures paid by Summit Investments on behalf of Red Rock Gathering	52			52
Ending balance at Dec. 31, 2013	1,201,737			232,594				566,532			379,287	23,324
Increase (Decrease) in Partners' Capital [Roll Forward]
Net (loss) income	-21,164			2,828				-15,948			-11,169	3,125
Unit-based compensation	4,696							4,696
Net assets retained by the Predecessor	0
Contribution from general partner	4,235											4,235
Purchase of Red Rock Gathering	-307,941			-307,941
Excess of purchase price over acquired carrying value of Red Rock Gathering				66,124				-37,910			-26,891	-1,323
Assets contributed to Red Rock Gathering from Summit Investments	4,233							2,426			1,722	85
Issuance of common units, net of offering costs	197,806							197,806
Distributions	-122,224							-67,658			-49,796	-4,770
Tax withholdings on vested SMLP LTIP awards	-656							-656
Contribution of net assets from Summit Investments in excess of consideration paid for Bison Midstream	0
Cash advance from Summit Investments to contributed subsidiaries, net	1,982			1,982
Capitalized interest allocated to Red Rock Gathering projects from Summit Investments	0
Expenses paid by Summit Investments on behalf of contributed subsidiaries	4,413			4,413
Capital expenditures paid by Summit Investments on behalf of Red Rock Gathering	0
Repurchase of SMLP LTIP units	-228								-228
Ending balance at Dec. 31, 2014	$966,889			$0				$649,060			$293,153	$24,676

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net (loss) income	($21,164,000)	$53,304,000	$42,997,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	83,934,000	70,994,000	36,866,000
Amortization of deferred loan costs	2,770,000	2,246,000	1,458,000
Unit-based compensation	4,696,000	3,506,000	1,876,000
Loss on asset sales, net	442,000	113,000	0
Goodwill impairment	54,199,000	0	0
Long-lived asset impairment	5,505,000	0	0
Purchase accounting adjustment	-1,185,000	0	0
Pay-in-kind interest on promissory notes payable to Sponsors	0	0	5,426,000
Changes in operating assets and liabilities:
Accounts receivable	-15,579,000	-18,605,000	-8,174,000
Due to/from affiliate	-883,000	1,427,000	-773,000
Trade accounts payable	-970,000	-3,419,000	-2,536,000
Change in deferred revenue	26,378,000	16,685,000	9,994,000
Ad valorem taxes payable	342,000	-11,000	3,125,000
Accrued interest	6,714,000	12,128,000	-484,000
Other, net	2,736,000	2,321,000	-383,000
Net cash provided by operating activities	147,935,000	140,689,000	89,392,000
Cash flows from investing activities:
Capital expenditures	-128,325,000	-109,376,000	-77,296,000
Proceeds from asset sales	325,000	585,000	0
Acquisition of gathering systems	-10,872,000	-210,000,000	0
Acquisition of gathering systems from affiliate	-305,000,000	-200,000,000	0
Net cash used in investing activities	-443,872,000	-518,791,000	-77,296,000
Cash flows from financing activities:
Distributions to unitholders	-122,224,000	-90,196,000	0
Borrowings under revolving credit facility	237,295,000	380,950,000	213,000,000
Repayments under revolving credit facility	-315,295,000	-294,180,000	-160,770,000
Deferred loan costs	-5,320,000	-10,608,000	-3,344,000
Tax withholdings on vested SMLP LTIP awards	-656,000	0	0
Proceeds from issuance of common units, net	197,806,000	0	263,125,000
Contribution from general partner	4,235,000	2,229,000	0
Cash advance from Summit Investments to contributed subsidiaries, net	1,982,000	738,000	500,000
Expenses paid by Summit Investments on behalf of contributed subsidiaries	4,413,000	10,149,000	2,536,000
Issuance of senior notes	300,000,000	300,000,000	0
Issuance of units to affiliate in connection with the Mountaineer Acquisition	0	100,000,000	0
Repurchase of equity-based compensation awards	-228,000	-11,957,000	0
Red Rock Gathering cash contributed by Summit Investments	0	0	1,097,000
Repayment of promissory notes payable to Sponsors	0	0	-209,230,000
Distributions to Sponsors	0	0	-123,138,000
Net cash provided by (used in) financing activities	302,008,000	387,125,000	-16,224,000
Net change in cash and cash equivalents	6,071,000	9,023,000	-4,128,000
Cash and cash equivalents, beginning of period	20,357,000	11,334,000	15,462,000
Cash and cash equivalents, end of period	26,428,000	20,357,000	11,334,000
Supplemental Cash Flow Disclosures:
Cash interest paid	31,524,000	9,016,000	8,283,000
Less: capitalized interest	3,172,000	4,705,000	2,784,000
Interest paid (net of capitalized interest)	28,352,000	4,311,000	5,499,000
Cash paid for taxes	0	660,000	650,000
Noncash Investing and Financing Activities:
Capital expenditures in trade accounts payable (period-end accruals)	6,359,000	16,470,000	8,523,000
Excess of purchase price over acquired carrying value of Red Rock Gathering	66,124,000	0	0
Red Rock Gathering working capital adjustment	-2,941,000	0	0
Assets contributed to Red Rock Gathering from Summit Investments	4,233,000	0	0
Issuance of units to affiliate to partially fund the Bison Drop Down	0	48,914,000	0
Contribution of net assets from Summit Investments in excess of consideration paid for Bison Midstream	0	56,535,000	0
Capitalized interest allocated to Red Rock Gathering projects from Summit Investments	0	496,000	0
Capital expenditures paid by Summit Investments on behalf of Red Rock Gathering	0	52,000	0
Pay-in-kind interest on promissory notes payable to Sponsors	0	0	6,337,000
Net assets retained by the Predecessor	0	0	4,417,000
Deferred initial public offering costs in trade accounts payable	$0	$0	$743,000

ORGANIZATION_BUSINESS_OPERATIO

ORGANIZATION, BUSINESS OPERATIONS AND BASIS OF PRESENTATION	12 Months Ended
ORGANIZATION, BUSINESS OPERATIONS AND BASIS OF PRESENTATION	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION, BUSINESS OPERATIONS AND BASIS OF PRESENTATION	ORGANIZATION, BUSINESS OPERATIONS AND BASIS OF PRESENTATION
	Organization. Summit Midstream Partners, LP ("SMLP" or the "Partnership"), a Delaware limited partnership, was formed in May 2012 and began operations in October 2012 in connection with its initial public offering ("IPO") of common limited partner units. SMLP is a growth-oriented limited partnership focused on developing, owning and operating midstream energy infrastructure assets that are strategically located in the core producing areas of unconventional resource basins, primarily shale formations, in North America.
	Effective with the completion of its IPO on October 3, 2012, SMLP had a 100% ownership interest in Summit Midstream Holdings, LLC ("Summit Holdings") which had a 100% ownership interest in both DFW Midstream Services LLC ("DFW Midstream") and Grand River Gathering, LLC ("Grand River Gathering" or the "Legacy Grand River" system).
	On June 4, 2013, the Partnership acquired all of the membership interests of Bison Midstream, LLC ("Bison Midstream") from a wholly owned subsidiary of Summit Midstream Partners, LLC ("Summit Investments") (the "Bison Drop Down"), and thereby acquired certain associated natural gas gathering pipeline, dehydration and compression assets in the Bakken Shale Play in Mountrail and Burke counties in North Dakota (the "Bison Gas Gathering system").
	Prior to the Bison Drop Down, on February 15, 2013, Summit Investments acquired Bear Tracker Energy, LLC ("BTE"), which was subsequently renamed Meadowlark Midstream Company, LLC ("Meadowlark Midstream"). The Bison Gas Gathering system was carved out from Meadowlark Midstream in connection with the Bison Drop Down. As such, it was determined to be a transaction among entities under common control.
	On June 21, 2013, Mountaineer Midstream Company, LLC ("Mountaineer Midstream"), a newly formed, wholly owned subsidiary of the Partnership, acquired certain natural gas gathering pipeline and compression assets in the Marcellus Shale Play in Doddridge and Harrison counties, West Virginia from an affiliate of MarkWest Energy Partners, L.P. ("MarkWest") (the "Mountaineer Acquisition"). In December 2013, Mountaineer Midstream was merged into DFW Midstream.
	In October 2012, Summit Investments acquired ETC Canyon Pipeline, LLC ("Canyon") from a subsidiary of Energy Transfer Partners, L.P. The Canyon gathering and processing assets were contributed to Red Rock Gathering Company, LLC ("Red Rock Gathering"), a newly formed, wholly owned subsidiary of Summit Investments. Red Rock Gathering gathers and processes natural gas and natural gas liquids in the Piceance Basin in western Colorado and eastern Utah. On March 18, 2014, SMLP acquired all of the membership interests of Red Rock Gathering from a subsidiary of Summit Investments (the "Red Rock Drop Down"). As such, it was determined to be a transaction among entities under common control. Concurrent with the closing of the Red Rock Drop Down, SMLP contributed its interest in Red Rock Gathering to Grand River Gathering.
	Summit Investments is a Delaware limited liability company and the predecessor for accounting purposes of SMLP. Summit Investments was formed and began operations in September 2009. Through August 2011, Summit Investments was wholly owned by Energy Capital Partners II, LLC and its parallel and co-investment funds (collectively, "Energy Capital Partners"). In August 2011, Energy Capital Partners sold an interest in Summit Investments to a subsidiary of GE Energy Financial Services, Inc. ("GE Energy Financial Services"). In June 2014, GE Energy Financial Services exchanged 100% of its Class A membership interests in Summit Investments for a new class of membership interests, structured as Class C Preferred interests. As a result, GE Energy Financial Services is no longer a Class A member of Summit Investments. Consequently, we refer to Energy Capital Partners and GE Energy Financial Services as our "Sponsors" for the period from August 17, 2011 until June 17, 2014, and we refer to Energy Capital Partners as our sole "Sponsor" subsequent to June 2014.
	In March 2013, Summit Investments contributed the ownership of its SMLP common and subordinated units along with its 2% general partner interest in SMLP (including the incentive distribution rights ("IDRs") in respect of SMLP) to Summit Midstream Partners Holdings, LLC ("SMP Holdings") in exchange for a continuing 100% interest in SMP Holdings. As of December 31, 2014, Summit Investments, through a wholly owned subsidiary, held 5,293,571 SMLP common units, all of our subordinated units, all of our general partner units representing a 2% general partner interest in SMLP and all of our IDRs.
	SMLP is managed and operated by the board of directors and executive officers of Summit Midstream GP, LLC (the "general partner"). Summit Investments, as the ultimate owner of our general partner, controls SMLP and has the right to appoint the entire board of directors of our general partner, including our independent directors. SMLP's operations are conducted through, and our operating assets are owned by, various wholly-owned operating subsidiaries. However, neither SMLP nor its subsidiaries have any employees. The general partner has the sole responsibility for providing the personnel necessary to conduct SMLP's operations, whether through directly hiring employees or by obtaining the services of personnel employed by others, including Summit Investments. All of the personnel that conduct SMLP's business are employed by the general partner and its subsidiaries, but these individuals are sometimes referred to as our employees.
	References to the "Company," "we," or "our," when used for dates or periods ended on or after the IPO, refer collectively to SMLP and its subsidiaries. References to the "Company," "we," or "our," when used for dates or periods ended prior to the IPO, refer collectively to Summit Investments and its subsidiaries.
	Initial Public Offering. On October 3, 2012, SMLP completed its IPO and the following transactions occurred:

	•	Summit Investments conveyed an interest in Summit Holdings to our general partner as a capital contribution;

	•	our general partner conveyed its interest in Summit Holdings to SMLP in exchange for (i) a continuation of its 2% general partner interest in SMLP, represented by 996,320 general partner units, and (ii) SMLP incentive distribution rights, or IDRs;

	•	Summit Investments conveyed its remaining interest in Summit Holdings to SMLP in exchange for (i) 10,029,850 common units (net of the impact of selling 1,875,000 common units to the public for cash in connection with the exercise of the underwriters’ option to purchase additional common units), representing a 20.1% limited partner interest in SMLP, (ii) 24,409,850 subordinated units, representing a 49.0% limited partner interest in SMLP, and (iii) the right to receive $88.0 million in cash as reimbursement for certain capital expenditures made with respect to the contributed assets;

	•	pursuant to its long-term incentive plan, SMLP granted 5,000 common units (in the aggregate) to two of its directors and 125,000 phantom units, with distribution equivalent rights, to certain employees;

	•	SMLP issued 14,375,000 common units to the public (including 1,875,000 additional common units sold out of the common units originally allocated to Summit Investments) representing a 28.9% limited partner interest in SMLP; and

	•	SMLP used the proceeds, net of underwriters’ fees, from the IPO of approximately $269.4 million to (i) repay $140.0 million outstanding under the revolving credit facility; (ii) make cash distributions to Summit Investments of (a) $88.0 million to reimburse Summit Investments for certain capital expenditures it incurred with respect to assets it contributed to us and (b) $35.1 million representing the funds received in connection with the underwriters exercising their option to purchase additional common units; and (iii) pay IPO expenses of approximately $6.3 million.
	Business Operations. We provide natural gas gathering, treating and processing services pursuant to primarily long-term and fee-based, natural gas gathering agreements with our customers. Our results are driven primarily by the volumes of natural gas that we gather, treat and process across our systems. Our gathering and processing systems and the unconventional resource basins in which they operate as of December 31, 2014 were as follows:

	•	Mountaineer Midstream – the Appalachian Basin, which includes the Marcellus Shale formation in northern West Virginia;

	•	Bison Midstream – the Williston Basin, which includes the Bakken and Three Forks shale formations in northwestern North Dakota;

	•	DFW Midstream – the Fort Worth Basin, which includes the Barnett Shale formation in north-central Texas; and

	•	Grand River Gathering – the Piceance Basin, which includes the Mesaverde formation and the Mancos and Niobrara shale formations in western Colorado and eastern Utah.
	Our operating subsidiaries are DFW Midstream (which includes the Mountaineer Midstream gathering system), Bison Midstream and Grand River Gathering. All of our operating subsidiaries are midstream energy companies focused on the development, construction and operation of natural gas gathering and processing systems.
	Basis of Presentation and Principles of Consolidation. We prepare our consolidated financial statements in accordance with accounting principles generally accepted in the United States of America ("GAAP"). These principles are established by the Financial Accounting Standards Board. We make estimates and assumptions that affect the reported amounts of assets and liabilities at the balance sheet dates, including fair value measurements, the reported amounts of revenue and expense, and the disclosure of contingencies. Although management believes these estimates are reasonable, actual results could differ from its estimates.
	We conduct our operations in the midstream sector through four reportable segments:

	•	the Marcellus Shale, which is served by Mountaineer Midstream;

	•	the Williston Basin, which is served by Bison Midstream;

	•	the Barnett Shale, which is served by DFW Midstream; and

	•	the Piceance Basin, which is served by Grand River. Grand River is composed of the Legacy Grand River and Red Rock Gathering systems.
	The assets of our reportable segments consist of natural gas gathering and processing systems and related plant and equipment. Our reportable segments reflect the way in which we internally report the financial information used to make decisions and allocate resources in connection with our operations.
	For the purposes of the consolidated financial statements, SMLP's results of operations reflect the Partnership's operations subsequent to the IPO and the results of the Predecessor for the period prior to the IPO. The consolidated financial statements also reflect the results of operations of: (i) Red Rock Gathering since October 23, 2012, (ii) Bison Midstream since February 16, 2013 and (iii) Mountaineer Midstream since June 22, 2013. SMLP recognized its acquisitions of Red Rock Gathering and Bison Midstream at Summit Investments' historical cost because the acquisitions were executed by entities under common control. The excess of the purchase price paid by SMLP over Summit Investments' net investment in Red Rock Gathering was recognized as a reduction to partners' capital. The excess of Summit Investments' net investment in Bison Midstream over the purchase price paid by SMLP was recognized as an addition to partners' capital. Due to the common control aspect, the Red Rock Drop Down and the Bison Drop Down were accounted for by the Partnership on an “as-if pooled” basis for the periods during which common control existed. The consolidated financial statements include the assets, liabilities, and results of operations of SMLP or the Predecessor and their respective wholly owned subsidiaries. All intercompany transactions among the consolidated entities have been eliminated in consolidation.

SUMMARY_OF_SIGNIFICANT_ACCOUNT

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Cash and Cash Equivalents. Cash and cash equivalents include temporary cash investments with original maturities of three months or less.

Accounts Receivable. Accounts receivable relate to gathering and other services provided to our natural gas producer customers and other counterparties. To the extent we doubt the collectability of our accounts receivable, we recognize an allowance for doubtful accounts. We did not experience any non-payments during the three-year period ended December 31, 2014. As a result, we did not recognize an allowance for doubtful accounts as of December 31, 2014 and 2013.

Other Current Assets. Other current assets primarily consist of the current portion of prepaid expenses that are charged to expense over the period of benefit or the life of the related contract.

Property, Plant, and Equipment. We record property, plant, and equipment at historical cost of construction or fair value of the assets at acquisition. We capitalize expenditures that extend the useful life of an asset or enhance its productivity or efficiency from its original design over the expected remaining period of use. For maintenance and repairs that do not add capacity or extend the useful life of an asset, we recognize expenditures as an expense as incurred. We capitalize project costs incurred during construction, including interest on funds borrowed to finance the construction of facilities, as construction in progress. Prior to the Red Rock Drop Down, a subsidiary of Summit Investments incurred interest expense related to certain Red Rock Gathering capital projects. The associated interest expense was allocated to Red Rock Gathering as a noncash equity contribution and capitalized into the basis of the asset.

We base an asset’s carrying value on estimates, assumptions and judgments for useful life and salvage value. We record depreciation on a straight-line basis over an asset’s estimated useful life. We base our estimates for useful life on various factors including age (in the case of acquired assets), manufacturing specifications, technological advances, and historical data concerning useful lives of similar assets.

Upon sale, retirement or other disposal, we remove the carrying value of an asset and its accumulated depreciation from our balance sheet and recognize the related gain or loss, if any.

Accrued capital expenditures are reflected in trade accounts payable.

Asset Retirement Obligations. We record a liability for asset retirement obligations only if and when a future asset retirement obligation with a determinable life is identified. As of December 31, 2014 and 2013, we evaluated whether any future asset retirement obligations existed. For identified asset retirement obligations, we then evaluated whether the expected retirement date and the related costs of retirement could be estimated. In performing this evaluation, we concluded that our natural gas gathering and processing assets have an indeterminate life because they are owned and will operate for an indeterminate future period when properly maintained. Because we did not have sufficient information to reasonably estimate the amount or timing of such obligations and we have no current plan to discontinue use of any significant assets, we did not provide for any asset retirement obligations as of December 31, 2014 or 2013.

Intangible Assets and Noncurrent Liability. Upon the acquisition of DFW Midstream, certain of our gas gathering contracts were deemed to have above-market pricing structures while another was deemed to have pricing that was below market. We have recognized the contracts that were above market at acquisition as favorable gas gathering contracts. We have recognized the contract that was deemed to be below market as a noncurrent liability. We amortize these intangibles on a units-of-production basis over the estimated useful life of the contract. We define useful life as the period over which the contract is expected to contribute directly or indirectly to our future cash flows. The related contracts have original terms ranging from 10 years to 20 years. We recognize the amortization expense associated with these intangible assets and liability in amortization of favorable and unfavorable contracts.

For our other gas gathering contracts, we amortize contract intangible assets over the period of economic benefit based upon the expected revenues over the life of the contract. The useful life of these contracts ranges from 10 years to 25 years. We recognize the amortization expense associated with these intangible assets in depreciation and amortization expense.

We have right-of-way intangible assets associated with city easements and easements granted within existing rights-of-way. We amortize these intangible assets over the shorter of the contractual term of the rights-of-way or the estimated useful life of the gathering system. The contractual terms of the rights-of-way range from 20 years to 30 years. The estimated useful life of our gathering systems is 30 years. We recognize the amortization expense associated with these intangible assets in depreciation and amortization expense.

Impairment of Long-Lived Assets. We test assets for impairment when events or circumstances indicate that the carrying value of a long-lived asset may not be recoverable. The carrying value of a long-lived asset is not recoverable if it exceeds the sum of the undiscounted cash flows expected to result from its use and eventual disposition. If we conclude that an asset’s carrying value will not be recovered through future cash flows, we recognize an impairment loss on the long-lived asset equal to the amount by which the carrying value exceeds its fair value. We determine fair value using an income approach in which we discount the asset’s expected future cash flows to reflect the risk associated with achieving the underlying cash flows. During the three-year period ended December 31, 2014, we concluded that none of our long-lived assets had been impaired, except as discussed in Notes 4 and 5.

Goodwill. Goodwill represents consideration paid in excess of the fair value of the net identifiable assets acquired in a business combination. We evaluate goodwill for impairment annually on September 30. We also evaluate goodwill whenever events or circumstances indicate that it is more likely than not that the fair value of a reporting unit is less than its carrying amount.

We test goodwill for impairment using a two-step quantitative test. In the first step, we compare the fair value of the reporting unit to its carrying value, including goodwill. If the reporting unit’s fair value exceeds its carrying amount, we conclude that the goodwill of the reporting unit has not been impaired and no further work is performed. If we determine that the reporting unit’s carrying value exceeds its fair value, we proceed to step two. In step two, we compare the carrying value of the reporting unit to its implied fair value. If we determine that the carrying amount of a reporting unit's goodwill exceeds its implied fair value, we recognize the excess of the carrying value over the implied fair value as an impairment loss.

Other Noncurrent Assets. Other noncurrent assets primarily consist of external costs incurred in connection with the issuance of our senior notes and the closing of our revolving credit facility and related amendments. We capitalize and then amortize these deferred loan costs over the life of the respective debt instrument. We recognize amortization of deferred loan costs in interest expense.

Derivative Contracts. We have commodity price exposure related to our sale of the physical natural gas we retain from our DFW Midstream customers, and our procurement of electricity to operate our electric-drive compression assets on the DFW Midstream system. Our gas gathering agreements with our DFW Midstream customers permit us to retain a certain quantity of natural gas that we gather to offset the power costs we incur to operate our electric-drive compression assets. We manage this direct exposure to natural gas and power prices through the use of forward power purchase contracts with wholesale power providers that require us to purchase a fixed quantity of power at a fixed heat rate based on prevailing natural gas prices on the Waha Hub Index. Because we also sell our retainage gas at prices that are based on the Waha Hub Index, we have effectively fixed the relationship between our compression electricity expense and natural gas retainage sales.

Accounting standards related to derivative instruments and hedging activities allow for normal purchase or sale elections and hedge accounting designations, which generally eliminate or defer the requirement for mark-to-market recognition in net income and thus reduce the volatility of net income that can result from fluctuations in fair values. We have designated these contracts as normal under the normal purchase and sale exception under the accounting standards for derivatives. We do not enter into risk management contracts for speculative purposes.

Fair Value of Financial Instruments. The fair-value-measurement standard under GAAP defines fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The standard characterizes inputs used in determining fair value according to a hierarchy that prioritizes those inputs based upon the degree to which the inputs are observable. The three levels of the fair value hierarchy are as follows:

•

Level 1. Inputs represent quoted prices in active markets for identical assets or liabilities;

•

Level 2. Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly (for example, quoted market prices for similar assets or liabilities in active markets or quoted market prices for identical assets or liabilities in markets not considered to be active, inputs other than quoted prices that are observable for the asset or liability, or market-corroborated inputs); and

•

Level 3. Inputs that are not observable from objective sources, such as management’s internally developed assumptions used in pricing an asset or liability (for example, an internally developed present value of future cash flows model that underlies management's fair value measurement).

The carrying amount of cash and cash equivalents, accounts receivable, and accounts payable reported on the balance sheet approximates fair value due to their short-term maturities.

A summary of the estimated fair value of our debt financial instruments follows.

December 31, 2014

December 31, 2013

Carrying value

Estimated

Carrying value

Estimated

fair value (Level 2)

(In thousands)

Revolving credit facility

208,000

286,000

5.5% Senior notes

300,000

281,750

—

7.5% Senior notes

300,000

306,750

300,000

314,625

The revolving credit facility’s carrying value on the balance sheet is its fair value due to its floating interest rate. The fair value for the senior notes is based on an average of nonbinding broker quotes as of December 31, 2014 and December 31, 2013. The use of different market assumptions or valuation methodologies may have a material effect on the estimated fair value of the senior notes.

Nonfinancial assets and liabilities initially measured at fair value include those acquired and assumed in connection with third-party business combinations.

Commitments and Contingencies. We record accruals for loss contingencies when we determine that it is probable that a liability has been incurred and that such economic loss can be reasonably estimated. Such determinations are subject to interpretations of current facts and circumstances, forecasts of future events, and estimates of the financial impacts of such events.

Revenue Recognition. We generate the majority of our revenue from the natural gas gathering, treating and processing services that we provide to our natural gas producer customers. We also generate revenue from our marketing of natural gas and NGLs. We realize revenues by receiving fees from our producer customers or by selling the residue natural gas and NGLs.

We recognize revenue earned from fee-based gathering, treating and processing services in gathering services and other fees revenue. We also earn revenue from the sale of physical natural gas purchased from our customers under percentage-of-proceeds and keep-whole arrangements. These revenues are recognized in natural gas, NGLs and condensate sales and other with corresponding expense recognition in cost of natural gas and NGLs. We sell substantially all of the natural gas that we retain from our DFW Midstream customers to offset the power expenses of the electric-driven compression on the DFW Midstream system. We also sell condensate retained from our gathering services at Grand River Gathering. Revenues from the retainage of natural gas and condensate are recognized in natural gas, NGLs and condensate sales and other; the associated expense is included in operation and maintenance expense. Certain customers reimburse us for costs we incur on their behalf. We record costs incurred and reimbursed by our customers on a gross basis.

We recognize revenue when all of the following criteria are met: (i) persuasive evidence of an exchange arrangement exists, (ii) delivery has occurred or services have been rendered, (iii) the price is fixed or determinable, and (iv) collectability is reasonably assured.

We obtain access to natural gas and provide services principally under contracts that contain one or more of the following arrangements:

•

Fee-based arrangements. Under fee-based arrangements, we receive a fee or fees for one or more of the following services: natural gas gathering, treating, and/or processing. Fee-based arrangements include natural gas purchase arrangements pursuant to which we purchase natural gas at the wellhead, or other receipt points, at a settled price at the delivery point less a specified amount, generally the same as the fees we would otherwise charge for gathering of natural gas from the wellhead location to the delivery point. The margins earned are directly related to the volume of natural gas that flows through the system.

•

Percent-of-proceeds arrangements. Under percent-of-proceeds arrangements, we generally purchase natural gas from producers at the wellhead, or other receipt points, gather the wellhead natural gas through our gathering system, treat the natural gas, process the natural gas and/or sell the natural gas to a third party for processing. We then remit to our producers an agreed-upon percentage of the actual proceeds received from sales of the residue natural gas and NGLs. Certain of these arrangements may also result in returning all or a portion of the residue natural gas and/or the NGLs to the producer, in lieu of returning sales proceeds. The margins earned are directly related to the volume of natural gas that flows through the system and the price at which we are able to sell the residue natural gas and NGLs.

•

Keep-Whole. Under keep-whole arrangements, after processing we keep 100% of the NGLs produced, and the processed natural gas, or value of the natural gas, is returned to the producer. Since some of the natural gas is used and removed during processing, we compensate the producer for the amount of natural gas used and removed in processing by supplying additional natural gas or by paying an agreed-upon value for the natural gas utilized. These arrangements have commodity price exposure for us because the costs are dependent on the price of natural gas and the revenues are based on the price of NGLs.

Certain of our natural gas gathering or processing agreements provide for a monthly, quarterly or annual MVC. Under these MVCs, our customers agree to ship a minimum volume of natural gas on our gathering systems or to pay a minimum monetary amount over certain periods during the term of the MVC. A customer must make a shortfall payment to us at the end of the contract period if its actual throughput volumes are less than its MVC for that period. Certain customers are entitled to utilize shortfall payments to offset gathering fees in one or more subsequent periods to the extent that such customer's throughput volumes in subsequent periods exceed its MVC for that period.

We recognize customer billings for obligations under their MVCs as revenue when the obligations are billable under the contract and the customer does not have the right to utilize shortfall payments to offset gathering fees in excess of its MVCs in subsequent periods.

We record customer billings for obligations under their MVCs as deferred revenue when the customer has the right to utilize shortfall payments to offset gathering or processing fees in subsequent periods. We recognize deferred revenue under these arrangements in revenue once all contingencies or potential performance obligations associated with the related volumes have either (i) been satisfied through the gathering or processing of future excess volumes of natural gas, or (ii) expired (or lapsed) through the passage of time pursuant to the terms of the applicable natural gas gathering agreement.

We classify deferred revenue as a current liability for arrangements where the expiration of a customer's right to utilize shortfall payments is twelve months or less. We classify deferred revenue as noncurrent for arrangements where the expiration of the right to utilize shortfall payments and our estimate of its potential utilization is more than 12 months.

Unit-Based Compensation. For awards of unit-based compensation, we determine a grant date fair value and recognize the related compensation expense, in the statement of operations over the vesting period of the respective awards.

Income Taxes. Since we are structured as a partnership, we are generally not subject to federal and state income taxes, except as noted below. As a result, our unitholders or members are individually responsible for paying federal and state income taxes on their share of our taxable income. Net income or loss for financial statement purposes may differ significantly from taxable income reportable to our unitholders as a result of differences between the tax basis and the financial reporting basis of assets and liabilities and the taxable income allocation requirements under our partnership agreement.

In general, legal entities that are chartered, organized or conducting business in the state of Texas are subject to a franchise tax (the "Texas Margin Tax"). The Texas Margin Tax has the characteristics of an income tax because it is determined by applying a tax rate to a tax base that considers both revenues and expenses. Our financial statements reflect provisions for these tax obligations.

In 2014, the Company elected to apply changes to the determination of cost of goods sold for the Texas Margin Tax which permits the use of accelerated depreciation allowed for federal income tax purposes. As a result of this change, we recognized a $1.0 million deferred tax liability and current income tax expense for the year ended December 31, 2014 was reduced by $0.3 million. The associated deferred tax liability of $1.3 million is included in other noncurrent liabilities at December 31, 2014.

Earnings Per Unit ("EPU"). We present earnings or loss per limited partner unit only for periods subsequent to the IPO. Prior to the IPO, Summit Investments' members held membership interests and not units.

We determine EPU by dividing the net income or loss that is attributed, in accordance with the net income and loss allocation provisions of the partnership agreement, to the common and subordinated unitholders under the two-class method, after deducting the general partner's 2% interest in net income or loss and any payments to the general partner in connection with their IDRs, by the weighted-average number of common and subordinated units outstanding during the years ended December 31, 2014 and 2013, and the period from October 1, 2012 to December 31, 2012. Diluted earnings or loss per limited partner unit reflects the potential dilution that could occur if securities or other agreements to issue common units, such as unit-based compensation, were exercised, settled or converted into common units. When it is determined that potential common units resulting from an award subject to performance or market conditions should be included in the diluted earnings per limited partner unit calculation, the impact is reflected by applying the treasury stock method.

Comprehensive Income. Comprehensive income is the same as net income or loss for all periods presented.

Environmental Matters. We are subject to various federal, state and local laws and regulations relating to the protection of the environment. Although we believe that we are in material compliance with applicable environmental regulations, the risk of costs and liabilities are inherent in pipeline ownership and operation. Liabilities for loss contingencies, including environmental remediation costs, arising from claims, assessments, litigation, fines, and penalties and other sources are charged to expense when it is probable that a liability has been incurred and the amount of the assessment and/or remediation can be reasonably estimated. There are no such material liabilities in the accompanying financial statements at December 31, 2014 or 2013. However, we can provide no assurance that significant costs and liabilities will not be incurred by the Partnership in the future. We are currently not aware of any material contingent liabilities that exist with respect to environmental matters.

Recent Accounting Pronouncements. Accounting standard setters frequently issue new or revised accounting rules. We review new pronouncements to determine the impact, if any, on our financial statements. There are currently no recent pronouncements that have been issued that we believe will materially affect our financial statements, except as noted below.

In May 2014, the Financial Accounting Standards Board released a joint revenue recognition standard, Accounting Standards Update No. 2014-09 ("ASC Update 2014-09"). Under ASC Update 2014-09, revenue will be recognized under a five-step model: (i) identify the contract with the customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to performance obligations; and (v) recognize revenue when (or as) the partnership satisfies a performance obligation. This new standard is effective for fiscal years, and interim periods within those years, beginning after December 15, 2016, and interim and annual periods thereafter. Early adoption is not permitted. We are currently in the process of evaluating the impact of this update.

SEGMENT_INFORMATION

SEGMENT INFORMATION	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment Information	SEGMENT INFORMATION
	In the fourth quarter of 2014, we discontinued the aggregation of all of our operating segments. As of December 31, 2014, our reportable segments are the Marcellus Shale, the Williston Basin, the Barnett Shale and the Piceance Basin. The Piceance Basin reportable segment aggregates our Legacy Grand River and Red Rock Gathering operating segments. Each of our reportable segments provide midstream services in a specific geographic area. Corporate represents those revenues and expenses that are not specifically attributable to a reportable segment, not individually reportable, or that have not been allocated to our reportable segments. The accounting policies of the reportable segments and Corporate are the same as those described in the summary of significant accounting policies.
	The following table presents assets by reportable segment as of December 31.


		December 31,
		2014				2013
		(In thousands)
	Assets:
	Marcellus Shale	$	243,884			$	214,379

	Williston Basin	311,041				337,610

	Barnett Shale	428,935				431,578

	Piceance Basin	872,437				876,969

	Total reportable segment assets	1,856,297				1,860,536

	Corporate	38,577				23,203

	Total assets	$	1,894,874			$	1,883,739

	We assess the performance of our reportable segments based on segment adjusted EBITDA. We define segment adjusted EBITDA as total revenues less total costs and expenses; plus (i) other income excluding interest income, (ii) depreciation and amortization, (iii) adjustments related to MVC shortfall payments, (iv) impairments and (v) other noncash expenses or losses, less other noncash income or gains. Segment adjusted EBITDA excludes the effect of allocated corporate expenses, such as certain general and administrative expenses (including compensation-related expenses and professional services fees) interest expense and income tax expense.
	The following table presents segment adjusted EBITDA by reportable segment.


		Year ended December 31,
		2014				2013			2012
		(In thousands)
	Reportable segment adjusted EBITDA:
	Marcellus Shale	$	15,940			$	6,333

	Williston Basin	20,422				16,865

	Barnett Shale	60,528				69,473			$	63,670

	Piceance Basin	107,953				80,941			53,179

	Total reportable segment adjusted EBITDA	$	204,843			$	173,612		$	116,849

	The following table presents a reconciliation of income before income taxes to total reportable segment adjusted EBITDA.


		Year ended December 31,
		2014				2013			2012
		(In thousands)
	Reconciliation of Income Before Income Taxes to Segment Adjusted EBITDA:
	Income before income taxes	$	(20,533	)		$	54,033		$	43,679

	Add:
	Interest expense and affiliated interest expense	40,159				19,173			12,766

	Depreciation and amortization	82,990				69,962			36,674

	Amortization of favorable and unfavorable contracts	944				1,032			192

	Allocated corporate expenses	11,065				8,773			10,903

	Adjustments related to MVC shortfall payments	26,565				17,025			10,768

	Unit-based compensation	4,696				3,506			1,876

	Loss on asset sales, net	442				113			—

Goodwill impairment	54,199				—			—

Long-lived asset impairment	5,505				—			—

Less:
Interest income	4				5			9

Impact of purchase price adjustments	1,185				—			—

Total reportable segment adjusted EBITDA	$	204,843			$	173,612		$	116,849

The following table summarizes details by reportable segment for the years ended December 31.


	Year ended December 31,
	2014				2013			2012
	(In thousands)
Revenues:
Marcellus Shale	$	22,694			$	9,588

Williston Basin	62,454				50,735

Barnett Shale	93,001				105,324			$	93,453

Piceance Basin	152,537				127,273			81,961

Total reportable segments	330,686				292,920			175,414

Corporate	—				—			(991		)

Total revenues	$	330,686			$	292,920		$	174,423


Depreciation and amortization:
Marcellus Shale	$	7,648			$	3,998

Williston Basin	18,132				16,057

Barnett Shale	15,657				13,929			$	12,078

Piceance Basin	40,965				35,527			24,310

Total reportable segments	82,402				69,511			36,388

Corporate	588				451			286

Total depreciation and amortization	$	82,990			$	69,962		$	36,674


Other income:
Marcellus Shale	$	—			$	—

Williston Basin	—				—

Barnett Shale	—				—			$	—

Piceance Basin	1,185				—			—

Total reportable segments	1,185				—			—

Corporate	4				5			9

Total other income	$	1,189			$	5		$	9


Capital expenditures:
Marcellus Shale	$	33,866			$	1,822

Williston Basin	46,927				26,381

Barnett Shale	14,567				29,534			$	39,588

Piceance Basin	32,505				50,709			36,899

Total reportable segments	127,865				108,446			76,487

Corporate	460				930			809

Total capital expenditures	$	128,325			$	109,376		$	77,296

PROPERTY_PLANT_AND_EQUIPMENT_N

PROPERTY, PLANT, AND EQUIPMENT, NET	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
PROPERTY, PLANT, AND EQUIPMENT, NET	PROPERTY, PLANT, AND EQUIPMENT, NET
	Details on property, plant, and equipment, net were as follows:


		Useful lives (In years)		December 31,
				2014				2013
				(Dollars in thousands)
	Natural gas gathering and processing systems	30		$	881,235			$	744,359

	Compressor stations and compression equipment	30		410,906				380,000

	Construction in progress	n/a		33,177				83,765

	Other	15-Apr		27,345				21,304

	Total			1,352,663				1,229,428

	Less: accumulated depreciation			117,011				71,347

	Property, plant, and equipment, net			$	1,235,652			$	1,158,081

	During the fourth quarter of 2014, we reviewed certain property, plant and equipment balances associated with a compressor station project on our DFW Midstream system that was terminated and wrote off approximately $5.5 million of costs. The net impact of this action is reflected in long-lived asset impairment on the statement of operations. We also sold certain fixed assets during the fourth quarter of 2014. The net impact of these transactions is reflected in loss on asset sales, net on the statement of operations.
	Also during the fourth quarter of 2014, prices for natural gas, NGLs and crude oil continued to decline such that we identified a need to evaluate the goodwill associated with the Bison Midstream system. In connection with this evaluation, we also evaluated the property, plant and equipment and intangible assets associated with the Bison Midstream system for impairment and concluded that no impairment was necessary.
	Construction in progress is depreciated consistent with its applicable asset class once it is placed in service. Depreciation expense related to property, plant, and equipment and capitalized interest were as follows:


		Year ended December 31,
		2014				2013				2012
		(In thousands)
	Depreciation expense	$	45,734			$	36,745			$	22,422

	Capitalized interest	3,172				4,705				2,784

IDENTIFIABLE_INTANGIBLE_ASSETS

IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL	IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL
	Identifiable Intangible Assets and Noncurrent Liability. Identifiable intangible assets and the noncurrent liability, which are subject to amortization, were as follows:


		December 31, 2014
		Useful lives		Gross carrying amount				Accumulated amortization				Net
		(In years)
		(Dollars in thousands)
	Favorable gas gathering contracts	18.7		$	24,195			$	(8,056	)		$	16,139

	Contract intangibles	12.5		426,464				(75,713		)		350,751

	Rights-of-way	24.2		112,393				(12,417		)		99,976

	Total amortizable intangible assets			$	563,052			$	(96,186	)		$	466,866


	Unfavorable gas gathering contract	10		$	10,962			$	(5,385	)		$	5,577



		December 31, 2013
		Useful lives		Gross carrying amount				Accumulated amortization				Net
		(In years)
		(Dollars in thousands)
	Favorable gas gathering contracts	18.7		$	24,195			$	(6,315	)		$	17,880

	Contract intangibles	12.5		426,464				(43,158		)		383,306

	Rights-of-way	24.3		108,706				(7,715		)		100,991

	Total amortizable intangible assets			$	559,365			$	(57,188	)		$	502,177


	Unfavorable gas gathering contract	10		$	10,962			$	(4,588	)		$	6,374

	We recognized amortization expense in revenues as follows:


		Year ended December 31,
		2014				2013				2012
		(In thousands)
	Amortization expense – favorable gas gathering contracts	$	(1,741	)		$	(2,078	)		$	(1,715	)
	Amortization expense – unfavorable gas gathering contract	797				1,046				1,524

	Amortization of favorable and unfavorable contracts	$	(944	)		$	(1,032	)		$	(191	)
	During the fourth quarter of 2014, prices for natural gas and crude oil continued to decline such that we identified a need to evaluate the goodwill associated with the Bison Midstream system, as discussed below. In connection with this evaluation, we also evaluated the intangible assets and property, plant and equipment associated with the Bison Midstream system for impairment and concluded that no impairment was necessary.
	We recognized amortization expense in costs and expenses as follows:


		Year ended December 31,
		2014				2013				2012
		(In thousands)
	Amortization expense – contract intangibles	$	32,554			$	28,654			$	12,642

	Amortization expense – rights-of-way	4,702				4,563				1,610

	The estimated aggregate annual amortization of intangible assets and noncurrent liability expected to be recognized as of December 31, 2014 for each of the five succeeding fiscal years follows.


		Assets				Liability
		(In thousands)
	2015	$	41,882			$	698

2016	41,846				924

2017	40,697				1,047

2018	40,301				1,123

2019	40,247				957

Goodwill. Recorded goodwill is related to the original acquisitions of the Grand River Gathering, Bison Midstream and Mountaineer Midstream systems. A rollforward of goodwill by reportable segment and in total follows.


	Piceance Basin				Williston Basin				Marcellus Shale				Total
	(In thousands)
Goodwill, December 31, 2012	$	45,478			$	—			$	—			$	45,478

Goodwill recognized in connection with the Bison Drop Down	—				54,199				—				54,199

Goodwill preliminarily recognized in connection with the Mountaineer Acquisition	—				—				18,089				18,089

Goodwill adjustment recognized in connection with finalizing accounting for the Mountaineer Acquisition and other	—				—				(1,878		)		(1,878		)

Goodwill, December 31, 2013	45,478				54,199				16,211				115,888

Goodwill impairment (1)	—				(54,199		)		—				(54,199		)

Goodwill, December 31, 2014	$	45,478			$	—			$	16,211			$	61,689

__________
(1) Balance represents the cumulative goodwill impairment as of December 31, 2014.
Annual Impairment Evaluation. As discussed in Note 2, we evaluate goodwill for impairment annually on September 30. We performed our annual goodwill impairment testing as of September 30, 2014 using a combination of the income and market approaches. The results thereof follow:

•	We determined that the fair value of the Grand River Gathering reporting unit, as included in the Piceance Basin reportable segment, substantially exceeded its carrying value, including goodwill as of September 30, 2014.

•	We determined that the fair value of the Mountaineer Midstream reporting unit, as included in the Marcellus Shale reportable segment, substantially exceeded its carrying value, including goodwill as of September 30, 2014.

•	We determined that the fair value of the Bison Midstream reporting unit, as included in the Williston Basin reportable segment, exceeded its carrying value, including goodwill, as of September 30, 2014. However, it did not exceed its carrying value, including goodwill, by a substantial amount.

•	Because the fair values of these reporting units exceeded their carrying values, including goodwill, there were no associated impairments of goodwill in connection with our 2014 annual goodwill impairment test.
Fourth Quarter 2014 Goodwill Impairment. We also evaluate goodwill whenever events or circumstances indicate that it is more likely than not that the fair value of a reporting unit is less than its carrying value, including goodwill. During the latter part of the fourth quarter of 2014, the declines in prices for natural gas, NGLs and crude oil accelerated, negatively impacting producers in each of our areas of operation. As a result, we considered whether the goodwill associated with our Grand River Gathering, Mountaineer Midstream and Bison Midstream reporting units could have been impaired. Our assessments related to Grand River Gathering and Mountaineer Midstream did not result in an indication that the associated goodwill had been impaired.
Our assessment related to the Bison Midstream reporting unit did result in an indication that the associated goodwill could have been impaired. In connection therewith, we noted that a key Bison Midstream customer announced that it was delaying its previously announced drilling plans. The combined impact of (i) the price declines on revenues under its percent-of proceeds contracts and (ii) the Partnership's reduction in its forecasted volume assumption in response to the decline in our customer’s drilling plans increased the likelihood that the goodwill associated with the Bison Midstream reporting unit was impaired. As such, we concluded that a triggering event occurred during the fourth quarter of 2014 requiring that we test the goodwill associated with the Bison Midstream reporting unit for impairment.
To estimate the fair value of the Bison Midstream reporting unit, we utilized two valuation methodologies: the market approach and the income approach. Both of these approaches incorporate significant estimates and assumptions to calculate enterprise fair value for a reporting unit. The most significant estimates and assumptions inherent within these two valuation methodologies are:

•	determination of the weighted-average cost of capital;

•	the selection of guideline public companies;

•	market multiples;

•	weighting of the income and market approaches;

•	growth rates;

•	commodity prices; and

•	the expected levels of throughput volume gathered.
Changes in the above and other assumptions could materially affect the estimated amount of fair value for any of our reporting units.
The results of our step one goodwill impairment testing indicated that the fair value of the Bison Midstream reporting unit was below its carrying value, including goodwill. This result required that we perform step two of the goodwill impairment test. To perform step two, we first determined the fair values of the identifiable assets and liabilities. Significant assumptions utilized in the determination of the fair value of each reporting unit's individual assets and liabilities included the determination of discount rate and contributing asset charge utilized in our contract intangibles, expected levels of throughput volume and associated capital expenditures and commodity prices.
Our preliminary estimates of the fair values of the identified assets and liabilities calculated in the step two testing of the Bison Midstream reporting unit indicated that all of the associated goodwill had been impaired. As such, we recorded an estimated goodwill impairment of $54.2 million. This amount represents our best estimate of impairment pending the finalization of the fair value calculations, which we expect to finalize in the first quarter of 2015.
Our impairment determinations, in the context of (i) our annual impairment evaluation and (ii) our fourth quarter 2014 evaluation, involved significant assumptions and judgments, as discussed above. Differing assumptions regarding any of these inputs could have a significant effect on the various valuations. As such, the fair value measurements utilized within these models are classified as non-recurring Level 3 measurements in the fair value hierarchy because they are not observable from objective sources. Due to the volatility of the inputs used, we cannot predict the likelihood of any future impairment.

DEFERRED_REVENUE

DEFERRED REVENUE	12 Months Ended
	Dec. 31, 2014
Deferred Revenue Disclosure [Abstract]
Deferred Revenue	DEFERRED REVENUE
	The majority of our gas gathering agreements provide for a monthly, quarterly or annual MVC from our customers. If a customer's actual throughput volumes are less than its MVC for the applicable period, it must make a shortfall payment to us at the end of that contract month, quarter or year, as applicable. The amount of the shortfall payment is based on the difference between the actual throughput volume shipped or processed for the applicable period and the MVC for the applicable period, multiplied by the applicable gathering or processing fee. To the extent that a customer's actual throughput volumes are above or below its MVC for the applicable period, however, many of our gas gathering agreements contain provisions that can reduce or delay the cash flows that we expect to receive from our MVCs. These provisions include the following:

	•	To the extent that a customer's throughput volumes are less than its MVC for the applicable period and the customer makes a shortfall payment, it may be entitled to an offset in one or more subsequent periods to the extent that its throughput volumes in subsequent periods exceed its MVC for those periods. In such a situation, we would not receive gathering fees on throughput in excess of a customer's monthly or annual MVC (depending on the terms of the specific gas gathering agreement) to the extent that the customer had made a shortfall payment with respect to one or more preceding months or years (as applicable).

	•	To the extent that a customer's throughput volumes exceed its MVC in the applicable period, it may be entitled to apply the excess throughput against its aggregate MVC, thereby reducing the period for which its annual MVC applies. As a result of this mechanism, the weighted-average remaining period for which our MVCs apply will be less than the weighted-average of the original stated contract terms of our MVCs.

	•	To the extent that certain of our customers' throughput volumes exceed its MVC for the applicable period, there is a crediting mechanism that allows the customer to build a bank of credits that it can utilize in the future to reduce shortfall payments owed in subsequent periods, subject to expiration if there is no shortfall in subsequent periods. The period over which this credit bank can be applied to future shortfall payments varies, depending on the particular gas gathering agreement.
	A rollforward of current deferred revenue follows.


		Williston Basin		Barnett		Piceance		Total
				Shale		Basin		current
		(In thousands)
	Current deferred revenue, January 1, 2012	$	—	$	—	$	—	$	—

	Additions	—		865		—		865

	Current deferred revenue, December 31, 2012	—		865		—		865

	Additions	—		1,555		—		1,555

	Less: revenue recognized due to expiration	—		865		—		865

	Current deferred revenue, December 31, 2013	—		1,555		—		1,555

	Additions	—		2,610		—		2,610

	Less: revenue recognized due to expiration	—		1,555		—		1,555

	Less: revenue recognized due to usage	—		233		—		233

	Current deferred revenue, December 31, 2014	$	—	$	2,377	$	—	$	2,377

	A rollforward of noncurrent deferred revenue follows.


		Williston Basin		Barnett		Piceance		Total noncurrent
				Shale		Basin
		(In thousands)
	Noncurrent deferred revenue, January 1, 2012	$	—	$	—	$	1,770	$	1,770

	Additions	—		—		9,129		9,129

	Noncurrent deferred revenue, December 31, 2012	—		—		10,899		10,899

	Additions(1)	6,389		—		12,395		18,784

	Noncurrent deferred revenue, December 31, 2013	6,389		—		23,294		29,683

	Additions	10,743		—		14,813		25,556

	Noncurrent deferred revenue, December 31, 2014	$	17,132	$	—	$	38,107	$	55,239

	__________
	(1) Noncurrent includes amounts recognized in connection with the Bison Drop Down.
	As of December 31, 2014, accounts receivable included $13.1 million of shortfall billings related to MVC arrangements that can be utilized to offset gathering fees in subsequent periods. Noncurrent deferred revenue at December 31, 2014 represents amounts that provide certain customers the ability to offset their gathering fees over a period up to seven years to the extent that the customer's throughput volumes exceeds its MVC.

LONGTERM_DEBT

LONG-TERM DEBT	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
LONG-TERM DEBT	LONG-TERM DEBT
	Long-term debt consisted of the following:


		December 31,
		2014		2013
		(In thousands)
	Variable rate senior secured revolving credit facility (2.67% at December 31, 2014 and 2.42% at December 31, 2013) due November 2018	$	208,000	$	286,000

	5.50% Senior unsecured notes due August 2022	300,000		—

	7.50% Senior unsecured notes due July 2021	300,000		300,000

	Total long-term debt	$	808,000	$	586,000

	The aggregate amount of our debt maturities during each of the years after December 31, 2014 are as follows:


		Long-term debt
		(In thousands)
	2015	$	—

	2016	—

	2017	—

	2018	208,000

	2019	—

	Thereafter	600,000

	Total long-term debt	$	808,000

	Revolving Credit Facility. We have a senior secured revolving credit facility which allows for revolving loans, letters of credit and swingline loans (the "revolving credit facility"). The revolving credit facility has a $700.0 million borrowing capacity, matures in November 2018, and includes a $200.0 million accordion feature. It is secured by the membership interests of Summit Holdings and those of its subsidiaries. Substantially all of Summit Holdings' and its subsidiaries' assets are pledged as collateral under the revolving credit facility. The revolving credit facility, and Summit Holdings' obligations, are guaranteed by SMLP and each of its subsidiaries.
	Borrowings under the revolving credit facility bear interest at the London Interbank Offered Rate ("LIBOR") or an Alternate Base Rate ("ABR") plus an applicable margin ranging from 0.75% to 1.75% for ABR borrowings and 1.75% to 2.75% for LIBOR borrowings, with the commitment fee ranging from 0.30% to 0.50% in each case based on our relative leverage at the time of determination. At December 31, 2014, the applicable margin under LIBOR borrowings was 2.50%, the interest rate was 2.67% and the unused portion of the revolving credit facility totaled $492.0 million (subject to a commitment fee of 0.500%).
	The revolving credit agreement contains affirmative and negative covenants customary for credit facilities of its size and nature that, among other things, limit or restrict the ability to: (i) incur additional debt; (ii) make investments; (iii) engage in certain mergers, consolidations, acquisitions or sales of assets; (iv) enter into swap agreements and power purchase agreements; (v) enter into leases that would cumulatively obligate payments in excess of $30.0 million over any 12-month period; and (vi) prohibits the payment of distributions by Summit Holdings if a default then exists or would result therefrom, and otherwise limits the amount of distributions Summit Holdings can make. In addition, the revolving credit facility requires Summit Holdings to maintain a ratio of consolidated trailing 12-month earnings before interest, income taxes, depreciation and amortization ("EBITDA," as defined in the credit agreement) to net interest expense of not less than 2.5 to 1.0 (as defined in the credit agreement) and a ratio of total net indebtedness to consolidated trailing 12-month EBITDA of not more than 5.0 to 1.0, or not more than 5.5 to 1.0 for up to 270 days following certain acquisitions.
	As of December 31, 2014, we were in compliance with the covenants in the revolving credit facility. There were no defaults or events of default during the year ended December 31, 2014.
	Senior Notes. On July 15, 2014, Summit Holdings and its 100% owned finance subsidiary, Summit Midstream Finance Corp. ("Finance Corp.," together with Summit Holdings, the "Co-Issuers"), co-issued $300.0 million of 5.50% senior unsecured notes maturing August 15, 2022 (the "5.5% senior notes"). In June 2013, the Co-Issuers co-issued $300.0 million of 7.50% senior unsecured notes maturing July 1, 2021 (the "7.5% senior notes").
	SMLP and all of its subsidiaries other than the Co-Issuers (the "Guarantors") have fully and unconditionally and jointly and severally guaranteed the 5.5% senior notes and the 7.5% senior notes. SMLP has no independent assets or operations. Summit Holdings has no assets or operations other than its ownership of its wholly owned subsidiaries and activities associated with its borrowings under the revolving credit facility, the 5.5% senior notes and the 7.5% senior notes. Finance Corp. has no independent assets or operations and was formed for the sole purpose of being a co-issuer of certain of Summit Holdings' indebtedness, including the 5.5% senior notes and the 7.5% senior notes. There are no significant restrictions on the ability of SMLP or Summit Holdings to obtain funds from its subsidiaries by dividend or loan.
	5.5% Senior Notes. We will pay interest on the 5.5% senior notes semi-annually in cash in arrears on February 15 and August 15 of each year, commencing February 15, 2015. The 5.5% senior notes are senior, unsecured obligations and rank equally in right of payment with all of our existing and future senior obligations. The 5.5% senior notes are effectively subordinated in right of payment to all of our secured indebtedness, to the extent of the collateral securing such indebtedness. We used the proceeds from the issuance of the 5.5% senior notes to repay a portion of the balance outstanding under our revolving credit facility.
	At any time prior to August 15, 2017, the Co-Issuers may redeem up to 35% of the aggregate principal amount of the 5.5% senior notes at a redemption price of 105.500% of the principal amount of the 5.5% senior notes, plus accrued and unpaid interest, if any, to the redemption date, with an amount not greater than the net cash proceeds of certain equity offerings. On and after August 15, 2017, the Co-Issuers may redeem all or part of the 5.5% senior notes at a redemption price of 104.125% (with the redemption premium declining ratably each year to 100.000% on and after August 15, 2020), plus accrued and unpaid interest, if any. Debt issuance costs of $5.1 million, recognized in other noncurrent assets, are being amortized over the life of the senior notes.
	The 5.5% senior notes' indenture restricts SMLP’s and the Co-Issuers’ ability and the ability of certain of their subsidiaries to: (i) incur additional debt or issue preferred stock; (ii) make distributions, repurchase equity or redeem subordinated debt; (iii) make payments on subordinated indebtedness; (iv) create liens or other encumbrances; (v) make investments, loans or other guarantees; (vi) sell or otherwise dispose of a portion of their assets; (vii) engage in transactions with affiliates; and (viii) make acquisitions or merge or consolidate with another entity. These covenants are subject to a number of important exceptions and qualifications. At any time when the senior notes are rated investment grade by each of Moody’s Investors Service, Inc. and Standard & Poor’s Ratings Services and no default or event of default under the indenture has occurred and is continuing, many of these covenants will terminate.
	The 5.5% senior notes' indenture provides that each of the following is an event of default: (i) default for 30 days in the payment when due of interest on the 5.5% senior notes; (ii) default in the payment when due of the principal of, or premium, if any, on the 5.5% senior notes; (iii) failure by the Co-Issuers or SMLP to comply with certain covenants relating to mergers and consolidations, change of control or asset sales; (iv) failure by SMLP for 180 days after notice to comply with certain covenants relating to the filing of reports with the SEC; (v) failure by the Co-Issuers or SMLP for 30 days after notice to comply with any of the other agreements in the indenture; (vi) specified defaults under any mortgage, indenture or instrument under which there may be issued or by which there may be secured or evidenced any indebtedness for money borrowed by SMLP or any of its restricted subsidiaries (or the payment of which is guaranteed by SMLP or any of its restricted subsidiaries); (vii) failure by SMLP or any of its restricted subsidiaries to pay certain final judgments aggregating in excess of $20.0 million; (viii) except as permitted by the indenture, any guarantee of the senior notes shall cease for any reason to be in full force and effect or any guarantor, or any person acting on behalf of any guarantor, shall deny or disaffirm its obligations under its guarantee of the senior notes; and (ix) certain events of bankruptcy, insolvency or reorganization described in the indenture. In the case of an event of default as described in the foregoing clause (ix), all outstanding 5.5% senior notes will become due and payable immediately without further action or notice. If any other event of default occurs and is continuing, the trustee or the holders of at least 25% in principal amount of the then outstanding 5.5% senior notes may declare all the 5.5% senior notes to be due and payable immediately.
	As of December 31, 2014, we were in compliance with the covenants for the 5.5% senior notes. There were no defaults or events of default for the 5.5% senior notes during the period from issuance through December 31, 2014.
	7.5% Senior Notes. The 7.5% senior notes were sold within the United States only to qualified institutional buyers in reliance on Rule 144A under the Securities Act of 1933, as amended (the "Securities Act"), and outside the United States only to non-U.S. persons in reliance on Regulation S under the Securities Act.
	We pay interest on the 7.5% senior notes semi-annually in cash in arrears on January 1 and July 1 of each year. The 7.5% senior notes are senior, unsecured obligations and rank equally in right of payment with all of our existing and future senior obligations. The 7.5% senior notes are effectively subordinated in right of payment to all of our secured indebtedness, to the extent of the collateral securing such indebtedness. We used the proceeds from the issuance of the 7.5% senior notes to repay a portion of the balance outstanding under our revolving credit facility.
	Effective as of April 7, 2014, all of the holders of our 7.5% senior notes exchanged their unregistered senior notes and the guarantees of those notes for registered notes and guarantees. The terms of the registered senior notes are substantially identical to the terms of the unregistered senior notes, except that the transfer restrictions, registration rights and provisions for additional interest relating to the unregistered senior notes do not apply to the registered senior notes.
	At any time prior to July 1, 2016, the Co-Issuers may redeem up to 35% of the aggregate principal amount of the 7.5% senior notes at a redemption price of 107.500% of the principal amount of the 7.5% senior notes, plus accrued and unpaid interest, if any, to the redemption date, with an amount not greater than the net cash proceeds of certain equity offerings. On and after July 1, 2016, the Co-Issuers may redeem all or part of the 7.5% senior notes at a redemption price of 105.625% (with the redemption premium declining ratably each year to 100.000% on and after July 1, 2019), plus accrued and unpaid interest, if any. Debt issuance costs of $7.4 million, recognized in other noncurrent assets, are being amortized over the life of the senior notes.
	The 7.5% senior notes indenture restricts SMLP’s and the Co-Issuers’ ability and the ability of certain of their subsidiaries to: (i) incur additional debt or issue preferred stock; (ii) make distributions, repurchase equity or redeem subordinated debt; (iii) make payments on subordinated indebtedness; (iv) create liens or other encumbrances; (v) make investments, loans or other guarantees; (vi) sell or otherwise dispose of a portion of their assets; (vii) engage in transactions with affiliates; and (viii) make acquisitions or merge or consolidate with another entity. These covenants are subject to a number of important exceptions and qualifications. At any time when the senior notes are rated investment grade by each of Moody’s Investors Service, Inc. and Standard & Poor’s Ratings Services and no default or event of default under the indenture has occurred and is continuing, many of these covenants will terminate.
	The 7.5% senior notes indenture provides that each of the following is an event of default: (i) default for 30 days in the payment when due of interest on the 7.5% senior notes; (ii) default in the payment when due of the principal of, or premium, if any, on the 7.5% senior notes; (iii) failure by the Co-Issuers or SMLP to comply with certain covenants relating to mergers and consolidations, change of control or asset sales; (iv) failure by SMLP for 180 days after notice to comply with certain covenants relating to the filing of reports with the SEC; (v) failure by the Co-Issuers or SMLP for 30 days after notice to comply with any of the other agreements in the indenture; (vi) specified defaults under any mortgage, indenture or instrument under which there may be issued or by which there may be secured or evidenced any indebtedness for money borrowed by SMLP or any of its restricted subsidiaries (or the payment of which is guaranteed by SMLP or any of its restricted subsidiaries); (vii) failure by SMLP or any of its restricted subsidiaries to pay certain final judgments aggregating in excess of $20.0 million; (viii) except as permitted by the indenture, any guarantee of the senior notes shall cease for any reason to be in full force and effect or any guarantor, or any person acting on behalf of any guarantor, shall deny or disaffirm its obligations under its guarantee of the 7.5% senior notes; and (ix) certain events of bankruptcy, insolvency or reorganization described in the indenture. In the case of an event of default as described in the foregoing clause (ix), all outstanding 7.5% senior notes will become due and payable immediately without further action or notice. If any other event of default occurs and is continuing, the trustee or the holders of at least 25% in principal amount of the then outstanding 7.5% senior notes may declare all the 7.5% senior notes to be due and payable immediately.
	As of December 31, 2014, we were in compliance with the covenants for the 7.5% senior notes. There were no defaults or events of default during the year ended December 31, 2014.

PARTNERS_CAPITAL_AND_MEMBERHIP

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS	12 Months Ended
PARTNERS' CAPITAL AND MEMBERHIP INTERESTS	Dec. 31, 2014
Equity [Abstract]
PARTNERS' CAPITAL AND MEMBERHIP INTERESTS	PARTNERS' CAPITAL AND MEMBERSHIP INTERESTS
	Partners' Capital
	SMLP was formed in May 2012. Prior to the closing of its IPO on October 3, 2012, SMLP had no outstanding common or subordinated units or operations.
	A rollforward of the number of common limited partner, subordinated limited partner and general partner units follows.


		Common			Subordinated			General partner		Total
	Units, January 1, 2012	—			—			—		—
	Units, January 1, 2012	—			—			—		—
	Units issued to the public in connection with the IPO	14,380,000			—			—		14,380,000
	Units issued to the public in connection with the IPO	14,380,000			—			—		14,380,000
	Units issued to Summit Investments in connection with the IPO	10,029,850			24,409,850			996,320		35,436,020
		10,029,850			24,409,850			996,320		35,436,020
	Units issued under SMLP LTIP	2,577			—			—		2,577
	Units issued under SMLP LTIP	2,577			—			—		2,577
	Units, December 31, 2012	24,412,427			24,409,850			996,320		49,818,597
	Units, December 31, 2012	24,412,427			24,409,850			996,320		49,818,597
	Units issued to a subsidiary of Summit Investments in connection with the Bison Drop Down (1)	1,553,849			—			31,711		1,585,560
		1,553,849			—			31,711		1,585,560
	Units issued to a subsidiary of Summit Investments in connection with the Mountaineer Acquisition (1)	3,107,698			—			63,422		3,171,120
		3,107,698			—			63,422		3,171,120
	Units issued under SMLP LTIP	5,892			—			—		5,892
	Units issued under SMLP LTIP	5,892			—			—		5,892
	Units, December 31, 2013	29,079,866			24,409,850			1,091,453		54,581,169
	Units, December 31, 2013	29,079,866			24,409,850			1,091,453		54,581,169
	Units issued in connection with the March Equity 2014 Offering (1)	5,300,000			—			108,337		5,408,337
		5,300,000			—			108,337		5,408,337
	Units issued under SMLP LTIP (1)(2)	46,647			—			861		47,508
	Units issued under SMLP LTIP (1)(2)	46,647			—			861		47,508
	Units, December 31, 2014	34,426,513			24,409,850			1,200,651		60,037,014
	Units, December 31, 2014	34,426,513			24,409,850			1,200,651		60,037,014
	__________
	(1) Including issuance to general partner in connection with contributions made to maintain 2% general partner interest.
	(2) Units issued under SMLP LTIP in 2014 is net of 14,300 units withheld to meet minimum statutory tax withholding requirements.
	In March 2014, we completed an underwritten public offering of 10,350,000 common units at a price of $38.75 per unit, of which 5,300,000 common units were offered by the Partnership and 5,050,000 common units were offered by a subsidiary of Summit Investments, pursuant to an effective shelf registration statement on Form S-3 previously filed with the SEC. Concurrently, our general partner made a capital contribution to maintain its 2% general partner interest in SMLP. We used the proceeds from the primary offering and the general partner capital contribution to fund a portion of the purchase of Red Rock Gathering.
	In September 2014, a subsidiary of Summit Investments completed an underwritten public offering of 4,347,826 SMLP common units pursuant to an effective shelf registration statement on Form S-3 previously filed with the SEC. We did not receive any proceeds from this offering.
	See Notes 1, 10 and 15 for information on units issued (i) in connection with our IPO, (ii) under the SMLP LTIP plan, and (iii) to fund acquisitions.
	Red Rock Drop Down. On March 18, 2014, SMLP acquired 100% of the membership interests in Red Rock Gathering from a subsidiary of Summit Investments. In exchange for its $241.8 million net investment in Red Rock Gathering, SMLP paid total cash consideration of $307.9 million, including working capital adjustments. As a result of the excess of the purchase price over acquired carrying value of Red Rock Gathering, SMLP recognized a capital distribution to Summit Investments. The calculation of the capital distribution and its allocation to partners' capital follow (in thousands).


	Summit Investments' net investment in Red Rock Gathering					$	241,817
	Summit Investments' net investment in Red Rock Gathering					$	241,817
	Total cash consideration paid to a subsidiary of Summit Investments					307,941
						307,941
	Excess of purchase price over acquired carrying value of Red Rock Gathering					$	(66,124	)

	Allocation of capital distribution:
	General partner interest	$	(1,323	)
	Common limited partner interest	(37,910		)
	Subordinated limited partner interest	(26,891		)
	Partners' capital allocation					$	(66,124	)
	Bison Drop Down. On June 4, 2013, a subsidiary of Summit Investments entered into a purchase and sale agreement with SMLP whereby SMLP acquired the Bison Gas Gathering system. In exchange for its $305.4 million net investment in Bison Midstream, SMLP paid Summit Investments and the general partner total cash and unit consideration of $248.9 million. As a result of the contribution of net assets in excess of consideration, SMLP recognized a capital contribution from Summit Investments. The details of total cash and unit consideration as well as the calculation of the capital contribution and its allocation to partners' capital follow (dollars in thousands).


	Summit Investments' net investment in Bison Midstream					$	305,449
	Summit Investments' net investment in Bison Midstream					$	305,449
	Aggregate cash paid to Summit Investments	$	200,000
	Aggregate cash paid to Summit Investments	$	200,000
	Issuance of 1,553,849 SMLP common units to Summit Investments	47,936
		47,936
	Issuance of 31,711 SMLP general partner units to the general partner	978
		978
	Total consideration					248,914
	Total consideration					248,914
Summit Investments' contribution of net assets in excess of consideration					$	56,535
					$	56,535

Allocation of capital contribution:
General partner interest	$	1,131
General partner interest	$	1,131
Common limited partner interest	28,558
Common limited partner interest	28,558
Subordinated limited partner interest	26,846
Subordinated limited partner interest	26,846
Partners' capital allocation					$	56,535
Partners' capital allocation					$	56,535
The number of units issued to Summit Investments and the general partner in connection with the Bison Drop Down was calculated based on an assumed equity issuance of $50.0 million and the five-day volume-weighted-average price as of June 3, 2013 of $31.53 per unit. The units were then valued as of June 4, 2013 (the date of closing) using the June 4, 2013 closing price of SMLP's units of $30.85.
The general partner interest allocation was calculated based on a 2% general partner interest in the contribution of assets in excess of consideration given by SMLP to Summit Investments. Common and subordinated limited partner interests allocations were calculated as their respective percentages of total limited partner capital applied to the balance of the contribution by Summit Investments after giving effect to the general partner allocation.
Mountaineer Acquisition. We completed the acquisition of Mountaineer Midstream on June 21, 2013. The purchase price of $210.0 million was funded with $110.0 million of borrowings under SMLP’s revolving credit facility and the issuance for cash of $100.0 million of SMLP common units and general partner interests to a subsidiary of Summit Investments and the general partner. The allocation and valuation of units issued to partially fund the Mountaineer Acquisition follow (dollars in thousands).


Issuance of 3,107,698 SMLP common units to Summit Investments	$	98,000
	$	98,000
Issuance of 63,422 SMLP general partner units to the general partner	2,000
	2,000
Issuance of units in connection with the Mountaineer Acquisition	$	100,000
	$	100,000
Pursuant to a unit purchase agreement, the number of units issued to Summit Investments and the general partner in connection with the Mountaineer Acquisition was calculated based on an assumed equity issuance of $100.0 million and the five-day volume-weighted-average price as of June 3, 2013 of $31.53 per unit.
Subordination. The principal difference between our common units and subordinated units is that in any quarter during the subordination period, holders of the subordinated units are not entitled to receive any distribution of available cash until the common units have received the minimum quarterly distribution ("MQD," as defined below) plus any arrearages in the payment of the MQD from prior quarters. Subordinated units will not accrue arrearages for unpaid quarterly distributions or quarterly distributions less than the MQD. If we do not pay the MQD on our common units, our common unitholders will not be entitled to receive such payments in the future except during the subordination period. To the extent we have available cash in any future quarter during the subordination period in excess of the amount necessary to pay the MQD to holders of our common units, we will use this excess available cash to pay any distribution arrearages related to prior quarters before any cash distribution is made to holders of subordinated units. When the subordination period ends, all subordinated units will convert into common units on a one-for-one basis, and thereafter no common units will be entitled to arrearages.
The subordination period will end on the first business day after we have earned and paid at least $1.60 (the MQD on an annualized basis) on each outstanding common unit and subordinated unit and the corresponding distribution on the general partner's 2.0% interest for each of three consecutive, non-overlapping four-quarter periods ending on or after December 31, 2015.
Cash Distribution Policy
Our cash distribution policy, as expressed in our partnership agreement, may not be modified or repealed without amending our partnership agreement. Our partnership agreement requires that we distribute all of our available cash (as defined below) within 45 days after the end of each quarter to unitholders of record on the applicable record date. Our policy is to distribute to our unitholders an amount of cash each quarter that is equal to or greater than the MQD stated in our partnership agreement.
Minimum Quarterly Distribution. Our partnership agreement generally requires that we make a minimum quarterly distribution to the holders of our common units and subordinated units of $0.40 per unit, or $1.60 on an annualized basis, to the extent we have sufficient cash from our operations after the establishment of cash reserves and the payment of costs and expenses, including reimbursements of expenses to our general partner. The amount of distributions paid under our policy is subject to fluctuations based on the amount of cash we generate from our business and the decision to make any distribution is determined by our general partner, taking into consideration the terms of our partnership agreement.
Definition of Available Cash. Available cash generally means, for any quarter, all cash on hand at the end of that quarter:

•	less the amount of cash reserves established by our general partner at the date of determination of available cash for that quarter to:

•	provide for the proper conduct of our business (including reserves for our future capital expenditures and anticipated future debt service requirements);

•	comply with applicable law, any of our debt instruments or other agreements; or

•	provide funds for distributions to our unitholders and to our general partner for any one or more of the next four quarters (provided that our general partner may not establish cash reserves for distributions unless it determines that the establishment of reserves will not prevent us from distributing the minimum quarterly distribution on all common units and any cumulative arrearages on such common units for the current quarter);

•	plus, if our general partner so determines, all or any portion of the cash on hand on the date of determination of available cash for the quarter resulting from working capital borrowings made subsequent to the end of such quarter.
General Partner Interest and Incentive Distribution Rights. Our general partner is entitled to 2.0% of all distributions that we make prior to our liquidation. Our general partner has the right, but not the obligation, to contribute a proportionate amount of capital to us to maintain its current general partner interest. Our general partner's initial 2.0% interest in our distributions will be reduced if we issue additional units in the future and our general partner does not contribute a proportionate amount of capital to us to maintain its 2.0% general partner interest.
Our general partner also currently holds incentive distribution rights that entitle it to receive increasing percentage allocations, up to a maximum of 50.0% (as set forth in the chart below), of the cash we distribute from operating surplus in excess of $0.46 per unit per quarter. The maximum distribution includes distributions paid to our general partner on its 2.0% general partner interest and assumes that our general partner maintains its general partner interest at 2.0%. The maximum distribution does not include any distributions that our general partner may receive on any common or subordinated units that it owns.
Percentage Allocations of Available Cash. The following table illustrates the percentage allocations of available cash between the unitholders and our general partner based on the specified target distribution levels. The amounts set forth in the column Marginal Percentage Interest in Distributions are the percentage interests of our general partner and the unitholders in any available cash we distribute up to and including the corresponding amount in the column Total Quarterly Distribution Per Unit Target Amount. The percentage interests shown for our unitholders and our general partner for the minimum quarterly distribution are also applicable to quarterly distribution amounts that are less than the minimum quarterly distribution. The percentage interests set forth below for our general partner include its 2.0% general partner interest and assume that our general partner has contributed any additional capital necessary to maintain its 2.0% general partner interest, our general partner has not transferred its incentive distribution rights and that there are no arrearages on common units.


	Total quarterly distribution per unit target amount		Marginal percentage interest in distributions
	Total quarterly distribution per unit target amount		Unitholders		General partner
Minimum quarterly distribution	$0.40		98.00%		2.00%
First target distribution	$0.40 up to $0.46		98.00%		2.00%
Second target distribution	above $0.46 up to $0.50		85.00%		15.00%
Third target distribution	above $0.50 up to $0.60		75.00%		25.00%
Thereafter	above $0.60		50.00%		50.00%
Details of cash distributions declared to date follow.


Attributable to the		Payment date		Per-unit distribution				Cash paid to common unitholders			Cash paid to subordinated unitholders		Cash paid to general partner		Cash paid for IDRs		Total distribution
quarter ended		Payment date		Per-unit distribution				Cash paid to common unitholders			Cash paid to subordinated unitholders		Cash paid to general partner		Cash paid for IDRs		Total distribution
				(In thousands, except per-unit amounts)
December 31, 2012		February 14, 2013		$	0.41			$	10,009		$	10,008	$	408	$	—	$	20,425
December 31, 2012		February 14, 2013		$	0.41			$	10,009		$	10,008	$	408	$	—	$	20,425
March 31, 2013		May 15, 2013		0.42				10,253			10,252		418		—		20,923
March 31, 2013		May 15, 2013		0.42				10,253			10,252		418		—		20,923
June 30, 2013		August 14, 2013		0.435				12,647			10,618		475		—		23,740
June 30, 2013		August 14, 2013		0.435				12,647			10,618		475		—		23,740
September 30, 2013		November 14, 2013		0.46				13,377			11,229		502		—		25,108
September 30, 2013		November 14, 2013		0.46				13,377			11,229		502		—		25,108
December 31, 2013		February 14, 2014		0.48				13,958			11,717		528		163		26,366
December 31, 2013		February 14, 2014		0.48				13,958			11,717		528		163		26,366
March 31, 2014		May 15, 2014		0.5				17,211			12,205		607		360		30,383
March 31, 2014		May 15, 2014		0.5				17,211			12,205		607		360		30,383
June 30, 2014		August 14, 2014		0.52				17,900			12,693		639		721		31,953
June 30, 2014		August 14, 2014		0.52				17,900			12,693		639		721		31,953
September 30, 2014		November 14, 2014		0.54				18,589			13,181		670		1,082		33,522
September 30, 2014		November 14, 2014		0.54				18,589			13,181		670		1,082		33,522
On January 22, 2015, the board of directors of our general partner declared a distribution of $0.56 per unit for the quarterly period ended December 31, 2014. The distribution was paid on February 13, 2015 to unitholders of record at the close of business on February 6, 2015. SMLP allocated its distribution in accordance with the third target distribution level for distributions attributable to the quarter ended December 31, 2014.
Membership Interests
Summit Investments' Equity in Contributed Subsidiaries. Summit Investments' equity in contributed subsidiaries represents its position in the net assets of Red Rock Gathering and Bison Midstream that have been acquired by SMLP. The balance also reflects net income attributable to Summit Investments for Red Rock Gathering and Bison Midstream for the periods beginning on their respective acquisition dates by Summit Investments and ending on the dates they were acquired by the Partnership. During the years ended December 31, 2014 and December 31, 2013, net income was attributed to Summit Investments for (i) Red Rock Gathering for the period from January 1, 2014 to March 18, 2014, for the year ended December 31, 2013 and for the period from October 23, 2012 to December 31, 2012 and (ii) Bison Midstream for the period from February 16, 2013 to June 4, 2013. Although included in partners' capital, net income attributable to Summit Investments has been excluded from the calculation of EPU for the years ended December 31, 2014 and 2013 and for the period from October 1, 2012 to December 31, 2012.
Predecessor Membership Interests. Holders of membership interests in Summit Investments participate in distributions and exercise the other rights or privileges available to each entity under Summit Investments' Fourth Amended and Restated Limited Liability Operating Agreement (the "Summit LLC Agreement"). In accordance with the Summit LLC Agreement, capital accounts are maintained for Summit Investments’ members. The capital account provisions of the Summit LLC Agreement incorporate principles established for U.S. federal income tax purposes and as such are not comparable to the equity accounts reflected under GAAP in our consolidated financial statements.
The Summit LLC Agreement sets forth the calculation to be used in determining the amount and priority of cash distributions that its membership interest holders will receive. Capital contributions required under the Summit LLC Agreement are in proportion to the members' respective percentage ownership interests. The Summit LLC Agreement also contains provisions for the allocation of net earnings and losses to members. For purposes of maintaining partner capital accounts, the Summit LLC Agreement specifies that items of income and loss shall be allocated among the partners in accordance with their respective percentage interests.

EARNINGS_PER_UNIT

EARNINGS PER UNIT	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
EARNINGS PER UNIT	EARNINGS PER UNIT
	The following table presents details on EPU.


		Year ended December 31,
		2014			2013		2012 (1)
		(In thousands, except per-unit amounts)
	Net (loss) income	$	(21,164	)	$	53,304	$	42,997

	Less: net income attributable to the pre-IPO period	—			—		24,112

	Less: net income attributable to Summit Investments	2,828			9,720		1,271

	Net (loss) income attributable to SMLP	(23,992		)	43,584		17,614

	Less: net (loss) income attributable to general partner, including IDRs	3,125			1,035		352

	Net (loss) income attributable to limited partners	$	(27,117	)	$	42,549	$	17,262


	Numerator for basic and diluted EPU:
	Allocation of net (loss) income among limited partner interests:
	Net (loss) income attributable to common units	$	(16,324	)	$	23,227	$	8,632

	Net (loss) income attributable to subordinated units	(10,793		)	19,322		8,630

	Net (loss) income attributable to limited partners	$	(27,117	)	$	42,549	$	17,262


	Denominator for basic and diluted EPU:
	Weighted-average common units outstanding – basic	33,311			26,951		24,412

	Effect of nonvested phantom units	—			150		132

	Weighted-average common units outstanding – diluted	33,311			27,101		24,544


	Weighted-average subordinated units outstanding – basic and diluted	24,410			24,410		24,410


	(Loss) earnings per limited partner unit:
	Common unit – basic	$	(0.49	)	$	0.86	$	0.35

	Common unit – diluted	$	(0.49	)	$	0.86	$	0.35

	Subordinated unit – basic and diluted	$	(0.44	)	$	0.79	$	0.35

	__________
	(1) Calculated for the period from October 1, 2012 to December 31, 2012
	Our general partner was not entitled to receive incentive distributions for periods prior to the fourth quarter of 2013 based on the amount of the distributions declared per common and subordinated unit. During the year ended December 31, 2014, we excluded 231,875 units from the calculation of diluted loss per common unit because their impact was anti-dilutive. There were no units excluded from the calculation of diluted earnings per common unit as we did not have any anti-dilutive units for the year ended December 31, 2013 or for the period from October 1, 2012 to December 31, 2012.

UNITBASED_COMPENSATION

UNIT-BASED COMPENSATION	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
UNIT-BASED COMPENSATION	UNIT-BASED COMPENSATION
	SMLP Long-Term Incentive Plan. The SMLP Long-Term Incentive Plan (the "SMLP LTIP") provides for equity awards to eligible officers, employees, consultants and directors of our general partner and its affiliates, thereby linking the recipients' compensation directly to SMLP’s performance. The SMLP LTIP is administered by our general partner's board of directors, though such administration function may be delegated to a committee appointed by the board. A total of 5.0 million common units was reserved for issuance pursuant to and in accordance with the SMLP LTIP. As of December 31, 2014, approximately 4.6 million common units remained available for future issuance.
	The SMLP LTIP provides for the granting, from time to time, of unit-based awards, including common units, restricted units, phantom units, unit options, unit appreciation rights, distribution equivalent rights, profits interest units and other unit-based awards. Grants are made at the discretion of the board of directors or compensation committee of our general partner. The administrator of the SMLP LTIP may make grants under the SMLP LTIP that contain such terms, consistent with the SMLP LTIP, as the administrator may determine are appropriate, including vesting conditions. The administrator of the SMLP LTIP may, in its discretion, base vesting on the grantee's completion of a period of service or upon the achievement of specified financial objectives or other criteria or upon a change of control (as defined in the SMLP LTIP) or as otherwise described in an award agreement. Termination of employment prior to vesting will result in forfeiture of the awards, except in limited circumstances as described in the plan documents. Units that are canceled or forfeited will be available for delivery pursuant to other awards.
	The following table presents phantom and restricted unit activity:


		Units			Weighted-average grant date
					fair value
	Nonvested phantom and restricted units, January 1, 2012	—			$	—

	Phantom units granted	125,000			20

	Restricted units granted	6,558			20.23

	Nonvested phantom and restricted units, December 31, 2012	131,558			20

	Phantom units granted	155,330			26.33

	Restricted units granted	835			27.5

	Phantom units forfeited	(4,041	)		25.99

	Nonvested phantom and restricted units, December 31, 2013	283,682			23.41

	Phantom units granted	136,867			42.32

	Phantom and restricted units vested	(61,917	)		25.33

	Phantom units forfeited	(22,430	)		25.56

	Nonvested phantom units, December 31, 2014	336,202			$	30.61

	A phantom unit is a notional unit that entitles the grantee to receive a common unit upon the vesting of the phantom unit or on a deferred basis upon specified future dates or events or, in the discretion of the administrator, cash equal to the fair market value of a common unit. Distribution equivalent rights for each phantom unit provide for a lump sum cash amount equal to the accrued distributions from the grant date to be paid in cash upon the vesting date. A restricted unit is a common limited partner unit that is subject to a restricted period during which the unit remains subject to forfeiture.
	The phantom units granted in connection with the IPO vest on the third anniversary of the IPO. All other phantom units granted to date vest ratably over a three-year period. Grant date fair value is determined based on the closing price of our common units on the date of grant multiplied by the number of phantom units awarded to the grantee. Holders of all phantom units granted to date are entitled to receive distribution equivalent rights for each phantom unit, providing for a lump sum cash amount equal to the accrued distributions from the grant date of the phantom units to be paid in cash upon the vesting date. Upon vesting, phantom unit awards may be settled, at our discretion, in cash and/or common units, but the current intention is to settle all phantom unit awards with common units. The restricted units granted in 2013 and 2012 maintained the vesting provisions of the share-based compensation awards they replaced, each of which had an original vesting period of four years. See "—DFW Net Profits Interests" below for additional information.
	As of December 31, 2014, the unrecognized unit-based compensation related to the SMLP LTIP was $4.1 million. Incremental unit-based compensation will be recorded over the remaining vesting period of approximately 2.25 years. Due to the limited and immaterial forfeiture history associated with the grants under the SMLP LTIP, no forfeitures were assumed in the determination of estimated compensation expense.
	Unit-based compensation recognized in general and administrative expense related to awards under the SMLP LTIP was as follows:


		Year ended December 31,
		2014				2013			2012
		(In thousands)
	SMLP LTIP unit-based compensation	$	4,696			$	2,999		$	269

	DFW Net Profits Interests. In connection with the formation of DFW Midstream in 2009, up to 5% of DFW Midstream's total membership interests were authorized for issuance (the "DFW Net Profits Interests"). Grants were made in 2009 and 2010. Beginning in October 2012 and continuing into April 2013, we entered into a series of repurchases with the remaining seven holders of the then-outstanding DFW Net Profits Interests whereby we exchanged $12.2 million for their vested DFW Net Profits Interests and 7,393 SMLP restricted units for their unvested DFW Net Profits Interests. The repurchase prices were determined by valuing the vested and unvested net profits interests in relation to the enterprise value of DFW Midstream and represented fair value at the dates of repurchase. Upon the conclusion of these repurchase transactions, there were no remaining or outstanding DFW Net Profits Interests.
	The DFW Net Profits Interests participated in distributions upon time vesting and the achievement of certain distribution targets and were accounted for as compensatory awards. Each grant vested ratably over four years and provided for accelerated vesting in certain limited circumstances.
	We determined the fair value of the DFW Net Profits Interests with assistance from a third-party valuation expert. The DFW Net Profits Interests were valued utilizing an option pricing method, which modeled membership interests as call options on the underlying equity value of DFW Midstream and considered the rights and preferences of each class of equity to allocate a fair value to each class. A significant input of the option pricing method was the enterprise value of DFW Midstream. We estimated the enterprise value utilizing a combination of the income and market approaches. Additional significant inputs used in the option pricing method included the length of holding period, discount for lack of marketability and volatility. Information regarding the vested and nonvested DFW Net Profits Interests were as follows:


		Year ended December 31,
		2013						2012
		Percentage Interest			Weighted-average grant date fair value (per 1.0% of DFW Net Profits Interest)			Percentage Interest			Weighted-average grant date fair value (per 1.0% of DFW Net Profits Interest)
		(Dollars in thousands)
	Nonvested, beginning of period	0.038	%		$	1,650		1.75	%		$	306

	Repurchased	0.038	%		$	1,650		0	%		$	—

	Vested	0	%		$	—		1.644	%		$	256

	Forfeited	0	%		$	—		0.069	%		$	765

	Nonvested, end of period	0	%		$	—		0.038	%		$	1,650


	Vested, end of period	0	%		$	—		4.294	%		$	257

We recognized noncash compensation expense related to the DFW Net Profits Interests within general and administrative expense of $17 thousand for the year ended December 31, 2013 and $0.7 million for the year ended December 31, 2012.
SMP Net Profits Interests. In connection with the formation of Summit Investments in 2009, up to 7.5% of total membership interests were authorized for issuance. SMP Net Profits Interests participate in distributions upon time vesting and the achievement of certain distribution targets. The SMP Net Profits Interests are accounted for as compensatory awards. Additional SMP Net Profits Interests were granted through January 2012. All grants vest ratably over five years and provide for accelerated vesting in certain limited circumstances, including a qualifying termination following a change in control. As of December 31, 2012, 6.355% of SMP Net Profits Interests had been granted to certain members of management, and no SMP Net Profits Interests had been forfeited. The SMP Net Profits Interests were retained by the Predecessor and as such are not reflected in SMLP's financial statements subsequent to the IPO, except as noted below.
During the year ended December 31, 2013, Summit Investments allocated $0.5 million of its annual expense associated with the SMP Net Profits Interests to Red Rock Gathering. This amount is reflected in general and administrative expenses.
We determined the fair value of the SMP Net Profits Interests as of the respective grant dates with assistance from a third-party valuation expert. We valued the SMP Net Profits Interests utilizing an option pricing method, which modeled membership interests as call options on the underlying equity value of Summit Investments and considered the rights and preferences of each class of equity to allocate a fair value to each class.
A significant input of the option pricing method is the enterprise value of Summit Investments. We estimated enterprise value utilizing a combination of the income and market approaches. The income approach utilized the discounted cash flow method, whereby we applied a discount rate to estimated future cash flows of Summit Investments. Under the market approach, we applied trading multiples of the securities of publicly-traded peer companies to Summit Investments' estimated future cash flows.
Additional significant inputs used in the option pricing method included length of holding period, discount for lack of marketability and volatility. The length of holding period was primarily determined based upon our Sponsors' expectations as of the grant date. We estimated the discount for lack of marketability and volatility with assistance from a third-party valuation firm. We estimated the discount for lack of marketability using a protective put methodology. The protective put methodology consisted of estimating the cost to insure an investment in the SMP Net Profits Interests over the length of the holding period. We estimated the expected volatility of the SMP Net Profits Interests based on the historical and implied volatilities of the securities of publicly-traded peer companies. We estimated historical volatility based on daily stock price returns over a look-back period commensurate with the length of the holding period for each grant of SMP Net Profits Interests. We estimated implied volatility based on the average implied volatility of the publicly-traded peer companies using data from Standard & Poor's Capital IQ proprietary research tool. We based the expected volatility conclusions on consideration of both the historical and implied volatilities of the publicly-traded peer companies as of the various grant dates.
The inputs used in the option pricing method for the SMP Net Profits Interests granted during the year ended December 31, 2012 were as follows:


	January
	2012
	grant
Length of holding period restriction (In years)	2.93

Discount for lack of marketability	24	%
Volatility	37	%
Information regarding the amount and grant-date fair value of the vested and nonvested SMP Net Profits Interests for the period in which they were reflected in our financial results follows.


	Year ended December 31,
	2012
	Percentage Interest			Weighted-average grant date fair value (per 1.0% of SMP Net Profits Interest)
	(Dollars in thousands)
Nonvested, beginning of period	3.958	%		$	1,003

Granted	0.5	%		$	1,780

Vested	1.271	%		$	965

Nonvested, end of period (1)	3.187	%		$	1,140


Vested, end of period	3.168	%		$	788

__________
(1) Subsequent to the IPO, the vested and nonvested net profits interests are obligations of the Predecessor and not the Partnership
We recognized noncash compensation expense related to the SMP Net Profits Interests in general and administrative expense of $0.5 million for the year ended December 31, 2013 and $0.9 million for the year ended December 31, 2012. For the year ended December 31, 2013, the expense reflects amounts allocated to Red Rock Gathering by Summit Investments prior to the Red Rock Drop Down. For the year ended December 31, 2012, the expense reflects amounts attributable to the Predecessor prior to our IPO.

CONCENTRATIONS_OF_RISK

CONCENTRATIONS OF RISK	12 Months Ended
	Dec. 31, 2014
Risks and Uncertainties [Abstract]
CONCENTRATIONS OF RISK	CONCENTRATIONS OF RISK
	Financial instruments that potentially subject us to concentrations of credit risk consist of cash and accounts receivable. We maintain our cash in bank deposit accounts that frequently exceed federally insured limits. We have not experienced any losses in such accounts and do not believe we are exposed to any significant risk.
	Accounts receivable primarily comprise natural gas gathering, treating and processing services we provide to our customers and also the sale of natural gas liquids resulting from our processing services. This industry concentration has the potential to impact our overall exposure to credit risk, either positively or negatively, in that our customers may be similarly affected by changes in economic, industry or other conditions. We monitor the creditworthiness of our counterparties and can require letters of credit for receivables from counterparties that are judged to have substandard credit, unless the credit risk can otherwise be mitigated.
	Counterparties accounting for more than 10% of total revenues were as follows:


		Year ended December 31,
		2014		2013		2012
	Revenue:
	Counterparty A - Piceance Basin	22	%	19	%	27	%
	Counterparty B - Williston Basin	10	%	*		—	%

	Counterparty C - Williston Basin	*		*		—	%

	Counterparty D - Marcellus Shale	*		*		—	%

	Counterparty E - Barnett Shale	*		15	%	19	%

	Counterparty F - Barnett Shale	*		*		14	%

	__________
	* Less than 10%
	Counterparties accounting for more than 10% of total accounts receivable were as follows:


		December 31,
		2014		2013
	Accounts receivable:
	Counterparty A - Piceance Basin	29	%	37	%
	Counterparty B - Williston Basin	*		*

	Counterparty C - Williston Basin	14	%	*

	Counterparty D - Marcellus Shale	10	%	*

	Counterparty E - Barnett Shale	*		11	%

	Counterparty F - Barnett Shale	*		*

	__________
	* Less than 10%

RELATEDPARTY_TRANSACTIONS

RELATED-PARTY TRANSACTIONS	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
RELATED-PARTY TRANSACTIONS	RELATED-PARTY TRANSACTIONS
	Recent Acquisitions. See Notes 1, 8 and 15 for disclosure of the Red Rock Drop Down, the Bison Drop Down and the funding of those transactions.
	Reimbursement of Expenses from General Partner. Our general partner and its affiliates do not receive a management fee or other compensation in connection with the management of our business, but will be reimbursed for expenses incurred on our behalf. Under our partnership agreement, we reimburse our general partner and its affiliates for certain expenses incurred on our behalf, including, without limitation, salary, bonus, incentive compensation and other amounts paid to our general partner's employees and executive officers who perform services necessary to run our business. Our partnership agreement provides that our general partner will determine in good faith the expenses that are allocable to us. Due to affiliate on the consolidated balance sheet represents the payables to our general partner for expenses incurred by it and paid on our behalf.
	Expenses incurred by the general partner and reimbursed by us under our partnership agreement were as follows:


		Year ended December 31,
		2014		2013		2012
		(In thousands)
	Operation and maintenance expense	$	17,223	$	13,808	$	2,913

	General and administrative expense	18,843		17,362		3,661

	Expense Allocations. During the period from January 1, 2014 to March 18, 2014 and the year ended December 31, 2013, Summit Investments incurred interest expense which was related to capital projects at Red Rock Gathering. As such, the associated interest expense was allocated to Red Rock Gathering as a noncash contribution and capitalized into the basis of the asset.
	Certain of Summit Investments’ current and former employees received Class B membership interests, classified as net profits interests, in Summit Investments (the “Net Profits Interests”). The Net Profits Interests participate in distributions upon time vesting and the achievement of certain distribution targets to Class A members or higher priority vested Net Profits Interests. The Net Profits Interests were accounted for as compensatory awards.
	Summit Investments allocated a portion of the annual expense associated with the Net Profits Interests to Red Rock Gathering during the year ended December 31, 2013. This amount is reflected in general and administrative expenses in the statement of operations.
	Expenses Paid by Summit Investments on Behalf of Red Rock Gathering. Prior to the Red Rock Drop Down, Summit Investments incurred certain support expenses and capital expenditures on behalf of Red Rock Gathering during the years ended December 31, 2014 and 2013. These transactions were settled periodically through membership interests prior to the Red Rock Drop Down.
	Electricity Management Services Agreement. We entered into a consulting arrangement with EquiPower Resources Corp. to assist with managing DFW Midstream's electricity price risk. EquiPower Resources Corp. is an affiliate of Energy Capital Partners and is also the employer of a director of our general partner. Amounts paid for such services were as follows:


		Year ended December 31,
		2014		2013		2012
		(In thousands)
	Payments for electricity management consulting services	$	234	$	199	$	204

	The consulting arrangement terminated on December 31, 2014.
	Engineering Services Agreement. We entered into an engineering services arrangement with IPS Engineering/EPC. IPS Engineering/EPC is an affiliate of Energy Capital Partners. We paid $0.6 million for such services during the year ended December 31, 2014 and $0.2 million for such services during the year ended December 31, 2013.
	Promissory Notes Payable to Sponsors. In conjunction with the acquisition of Grand River Gathering in 2011, we executed $200.0 million of promissory notes, on an unsecured basis, with the Sponsors. The notes had an 8% interest rate and were scheduled to mature in October 2013. In May 2012, we borrowed $163.0 million under the revolving credit facility and used a portion of the same borrowings to prepay $160.0 million principal amount of the promissory notes payable to the Sponsors. Then in July 2012, we borrowed an additional $50.0 million under the revolving credit facility, a portion of which was used to pay the remaining $49.2 million principal amount of the promissory notes payable to Sponsors (inclusive of accrued pay-in-kind interest).
	In accordance with the terms of the underlying note agreement, prior to their repayment in July 2012, we elected to make all interest payments on the note in kind. The amount of interest paid in kind and accrued to the balance of the notes for year ended December 31, 2012, was approximately $6.3 million, of which we capitalized $0.9 million of interest expense related to costs incurred on capital projects under construction.
	Diligence Expenses. In the past, the Sponsors reimbursed Summit Investments for transactional due diligence expenses related to proposed transactions that were not completed. As of December 31, 2011, we had a receivable from the Sponsors of $1.3 million for similar expenses. During the year ended December 31, 2012, we were reimbursed $0.3 million, while $1.0 million was not paid.

BENEFIT_PLAN

BENEFIT PLAN	12 Months Ended
BENEFIT PLAN	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Benefit plan	BENEFIT PLAN
Benefit plan	We have a defined contribution benefit plan for our employees. The expense associated with this plan was approximately $0.8 million in 2014, $0.6 million in 2013, and $0.2 million in 2012.

COMMITMENTS_AND_CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES
	Operating Leases. We and Summit Investments lease certain office space to support our operations. We have determined that our leases are operating leases. We recognize total rent expense incurred or allocated to us in general and administrative expenses. Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows:


		Year ended December 31,
		2014		2013		2012

	Rent expense	$	1,677	$	1,381	$	732

	Future minimum lease payments for the Partnership's operating leases are immaterial.
	Legal Proceedings. The Partnership is involved in various litigation and administrative proceedings arising in the normal course of business. In the opinion of management, any liabilities that may result from these claims or those arising in the normal course of business would not individually or in the aggregate have a material adverse effect on its financial position or results of operations.

ACQUISITIONS

ACQUISITIONS	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Acquisitions	ACQUISITIONS
	Red Rock Gathering System. On March 18, 2014, the Partnership acquired Red Rock Gathering from a subsidiary of Summit Investments, subject to customary working capital adjustments. The Partnership paid total cash consideration of $307.9 million, comprising $305.0 million at the date of acquisition and $2.9 million of working capital adjustments that were recognized in due to affiliate as of December 31, 2014 and settled in February 2015. The acquisition of Red Rock Gathering was funded with the net proceeds from an offering of common units in March 2014, $100.0 million of borrowings under our revolving credit facility and cash on hand. Because of the common control aspects in the drop down transaction, the Red Rock Gathering acquisition was deemed a transaction between entities under common control and, as such, was accounted for on an “as-if pooled” basis for all periods in which common control existed. SMLP’s financial results retrospectively include Red Rock Gathering’s financial results for all periods ending after October 23, 2012, the date Summit Investments acquired its interests, and before March 18, 2014.
	Summit Investments acquired the natural gas gathering pipeline, dehydration, compression and processing assets in the Piceance Basin in western Colorado and eastern Utah that comprise the Red Rock Gathering system from a subsidiary of Energy Transfer Partners, L.P. in September 2012 for $206.7 million. Summit Investments' acquisition of the Red Rock Gathering system closed on October 23, 2012. Summit Investments accounted for its acquisition of Red Rock Gathering under the acquisition method of accounting. Red Rock Gathering's identifiable tangible and intangible assets acquired and liabilities assumed were recognized at their fair values as of October 23, 2012. The intangible assets that were acquired comprised right-of-way easements with a life of 20 years upon acquisition. Their fair values were determined based upon assumptions related to future cash flows, discount rates, asset lives, and projected capital expenditures to complete the Red Rock Gathering system. The final fair values of the assets acquired and liabilities assumed as of October 23, 2012, were as follows (in thousands):


	Red Rock Gathering purchase price				$	206,694

	Cash	$	1,097

	Accounts receivable	8,018

	Other assets	317

	Property, plant, and equipment	150,401

	Rights-of-way	52,197

	Other noncurrent assets	164

	Total assets acquired	212,194

	Trade accounts payable	2,558

	Other current liabilities	2,942

	Total liabilities assumed	$	5,500

	Net identifiable assets acquired				$	206,694

	During the fourth quarter of 2014, we identified and wrote off the balance associated with a working capital adjustment received after the purchase accounting measurement period closed for Summit Investments' acquisition of Red Rock Gathering. This write off was recognized as a $1.2 million increase to gathering services and other fees for the year ended December 31, 2014.
	Lonestar Assets. DFW Midstream completed the acquisition of certain natural gas gathering assets located in the Barnett Shale Play ("Lonestar") from Texas Energy Midstream, L.P. ("TEM") for $10.9 million on September 30, 2014. The Lonestar assets gather natural gas under two long-term, fee-based contracts. SMLP is accounting for the purchase under the acquisition method of accounting. As of September 30, 2014, we preliminarily assigned the full purchase price to property, plant and equipment. During the fourth quarter of 2014, we received additional information from TEM and finalized the purchase price allocation.
	Bison Gas Gathering System. On February 15, 2013, Summit Investments acquired BTE. On June 4, 2013, a subsidiary of Summit Investments entered into a purchase and sale agreement with SMLP whereby SMLP acquired the Bison Gas Gathering system. The Bison Gas Gathering system was carved out from Meadowlark Midstream and primarily gathers associated natural gas production from customers operating in Mountrail and Burke counties in North Dakota under long-term contracts ranging from five years to 15 years. The weighted-average life of the acquired contracts was 12 years upon acquisition.
	Summit Investments accounted for its purchase of BTE (the "BTE Transaction") under the acquisition method of accounting, whereby the various gathering systems' identifiable tangible and intangible assets acquired and liabilities assumed were recorded based on their fair values as of February 15, 2013. The intangible assets that were acquired are composed of gas gathering agreement contract values and rights-of-way easements. Their fair values were determined based upon assumptions related to future cash flows, discount rates, asset lives, and projected capital expenditures to complete the system.
	Because the Bison Drop Down was executed between entities under common control, SMLP recognized the acquisition of the Bison Gas Gathering system at historical cost which reflected Summit Investments fair value accounting for the BTE Transaction. Furthermore, due to the common control aspect, the Bison Drop Down was accounted for by SMLP on an “as-if pooled” basis for all periods in which common control existed. Common control began on February 15, 2013 concurrent with the BTE Transaction.
	The fair values of the assets acquired and liabilities assumed as of February 15, 2013, were as follows (in thousands):


	Purchase price assigned to Bison Gas Gathering system				$	303,168

	Current assets	$	5,705

	Property, plant, and equipment	85,477

	Intangible assets	164,502

	Other noncurrent assets	2,187

	Total assets acquired	257,871

	Current liabilities	6,112

	Other noncurrent liabilities	2,790

	Total liabilities assumed	$	8,902

	Net identifiable assets acquired				248,969

	Goodwill				$	54,199

	The Bison Drop Down closed on June 4, 2013. The total acquisition purchase price of $248.9 million was funded with $200.0 million of borrowings under SMLP’s revolving credit facility and the issuance of $47.9 million of SMLP common units to Summit Investments and $1.0 million of general partner interests to SMLP’s general partner. Summit Investments had a net investment in the Bison Gas Gathering system of $303.2 million and received total consideration of $248.9 million from SMLP. As a result, SMLP recognized a capital contribution from Summit Investments for the contribution of net assets in excess of consideration paid.
	Mountaineer Midstream. We completed the acquisition of Mountaineer Midstream from MarkWest for $210.0 million on June 21, 2013. The Mountaineer Midstream natural gas gathering and compression assets are located in the Appalachian Basin which includes the Marcellus Shale formation primarily in Doddridge and Harrison counties in northern West Virginia. The Mountaineer Midstream system consists of newly constructed, high-pressure gas gathering pipelines, certain rights-of-way associated with the pipeline, and two compressor stations. The assets gather natural gas under a long-term, fee-based contract with Antero Resources Corp. ("Antero"). The life of the acquired contract was 13 years upon acquisition.
	The Mountaineer Acquisition was funded with $110.0 million of borrowings under the Partnership's revolving credit agreement and the issuance of $100.0 million of common and general partner interests to a subsidiary of Summit Investments. For the year ended December 31, 2013, SMLP recorded $9.6 million of revenue and $2.3 million of net income related to Mountaineer Midstream.
	SMLP accounted for the Mountaineer Acquisition under the acquisition method of accounting. As of June 30, 2013, we preliminarily assigned the full $210.0 million purchase price to property plant and equipment. During the third quarter of 2013, we received additional information and, as a result, preliminarily assigned $158.3 million of the purchase price to property, plant and equipment, $27.1 million to contract intangibles, $6.5 million to rights-of-way and $18.1 million to goodwill. During the fourth quarter of 2013, we received additional information from MarkWest and finalized the purchase price allocation.
	The final fair values of the assets acquired and liabilities assumed as of June 21, 2013, were as follows (in thousands):


Purchase price assigned to Mountaineer Midstream				$	210,000

Property, plant, and equipment	$	163,661

Gas gathering agreement contract intangibles	24,019

Rights-of-way	6,109

Total assets acquired	193,789

Total liabilities assumed	$	—

Net identifiable assets acquired				193,789

Goodwill				$	16,211

Grand River Gathering. During the fourth quarter of 2014, we identified and wrote off certain balances previously recognized in connection with the Predecessor's purchase accounting for Grand River Gathering. This write off was recognized as a $1.2 million increase to other income.
Supplemental Disclosures – As-If Pooled Basis. As noted above, SMLP's acquisition of Red Rock Gathering and the Bison Gas Gathering system were transactions between commonly controlled entities which required that SMLP account for the acquisitions in a manner similar to a pooling of interests. As a result, the historical financial statements of the Partnership, Red Rock Gathering and the Bison Gas Gathering system have been combined to reflect the historical operations, financial position and cash flows from the date common control began. Revenues and net income for the previously separate entities and the combined amounts for the years ended December 31, 2014, 2013 and 2012, as presented in these consolidated financial statements follow.


	Year ended December 31,
	2014			2013			2012
	(In thousands)
SMLP revenues	$	319,373		$	225,192		$	165,499

Red Rock Gathering revenues	11,313			50,114			8,924

Bison Gas Gathering system revenues (1)				17,614

Combined revenues	$	330,686		$	292,920		$	174,423


SMLP net (loss) income	$	(23,992	)	$	43,584		$	41,726

Red Rock Gathering net income	2,828			9,668			1,271

Bison Gas Gathering system net income (1)				52

Combined net (loss) income	$	(21,164	)	$	53,304		$	42,997

__________
(1) Results are fully reflected in SMLP's results of operations for the year ended December 31, 2014.
Unaudited Pro Forma Financial Information. The following unaudited pro forma financial information assumes that:

•	The acquisition of the Bison Gas Gathering system occurred on January 1, 2012. The pro forma results for Bison Midstream were derived from revenues and net income in 2013 and 2012.

•	The acquisition of Mountaineer Midstream occurred on January 1, 2012. The pro forma results for Mountaineer Midstream were derived from revenues and net income in 2013. Mountaineer Midstream was not operational until November 2012.

•	The acquisition of Red Rock Gathering occurred on January 1, 2011. The pro forma results reflect actual Red Rock Gathering revenues and net income earned and recognized in 2014 and 2013 and by annualizing the actual operating results for Red Rock Gathering that were recorded in 2012 for the year ended December 31, 2012.

•	The acquisition of the Lonestar assets is immaterial for pro forma purposes and as such has not been reflected below.

•	Pro forma net income for the year ended December 31, 2014 has been adjusted to remove the impact of $0.7 million of nonrecurring transaction costs associated with the acquisition of Red Rock Gathering.

•	Pro forma net income for the year ended December 31, 2013 has been adjusted to remove the impact of $2.5 million of nonrecurring transaction costs associated with the acquisitions of Bison Midstream and Mountaineer Midstream.

•	Pro forma net income for the year ended December 31, 2012 has been adjusted to remove the impact of $1.6 million of nonrecurring transaction costs associated with the acquisition of Red Rock Gathering.

•	Pro forma adjustments in 2013 and 2012 also reflect the impact of $310.0 million of incremental borrowings on our revolving credit facility for the Bison Midstream and Mountaineer Midstream acquisitions and incremental depreciation and amortization expense associated with the acquired property, plant and equipment and contract intangibles as a result of the application of fair value accounting for Bison Midstream.

•	Pro forma adjustments in 2014, 2013 and 2012 also reflect the impact of a 5,300,000 common unit issuance, the general partner capital contribution to maintain its 2% general partner interest and $100.0 million of incremental borrowings on our revolving credit facility to fund the acquisition of Red Rock Gathering.


	Year ended December 31,
	2014			2013			2012
	(In thousands, except for per-unit amounts)
Total Red Rock Gathering revenues included in consolidated revenues	$	73,266		$	50,114		$	8,924

Total Bison Midstream and Mountaineer Midstream revenues included in consolidated revenues				60,323


Total Red Rock Gathering net income included in consolidated net income	$	27,447		$	9,668		$	1,271

Total Bison Midstream and Mountaineer Midstream net loss included in consolidated net income				(457		)

Pro forma total revenues	$	330,686		$	305,071		$	256,637

Pro forma net (loss) income	(20,938		)	47,371			38,639


Pro forma common EPU - basic and diluted	$	(0.41	)	$	0.79		$	0.28

Pro forma subordinated EPU - basic and diluted	(0.41		)	0.79			0.28

The unaudited pro forma financial information presented above is not necessarily indicative of (i) what our financial position or results of operations would have been if the acquisitions of Bison Midstream and Mountaineer Midstream had occurred on January 1, 2012 or if the acquisition of Red Rock Gathering had occurred on January 1, 2011, or (ii) what SMLP’s financial position or results of operations will be for any future periods.

UNAUDITED_QUARTERLY_FINANCIAL_

UNAUDITED QUARTERLY FINANCIAL DATA	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Data	UNAUDITED QUARTERLY FINANCIAL DATA
	Summarized information on the consolidated results of operations for each of the quarters during the two-year period ended December 31, 2014, follows.


		Quarter ended				Quarter ended			Quarter ended			Quarter ended
		31-Dec-14				30-Sep-14			June 30,			March 31,
									2014			2014
		(In thousands, except per-unit amounts)
	Total revenues	$	94,658			$	79,030		$	80,796		$	76,202


	Net (loss) income attributable to partners(1)	$	(37,686	)		$	6,113		$	4,036		$	3,545

	Less: net (loss) income attributable to general partner, including IDRs	689				1,204			801			431

	Net (loss) income attributable to limited partners	$	(38,375	)		$	4,909		$	3,235		$	3,114


	(Loss) earnings per limited partner unit:
	Common unit – basic	$	(0.65	)		$	0.08		$	0.05		$	0.08

	Common unit – diluted	$	(0.65	)		$	0.08		$	0.05		$	0.08

	Subordinated unit – basic and diluted	$	(0.65	)		$	0.08		$	0.05		$	0.02

	__________
	(1) In the quarter ended December 31, 2014, includes $54.2 million of goodwill impairment and $5.5 million of long-lived asset impairment.


		Quarter ended				Quarter ended			Quarter ended			Quarter ended
		31-Dec-13				30-Sep-13			June 30,			March 31,
									2013			2013
		(In thousands, except per-unit amounts)
	Total revenues(1)	$	83,455			$	76,019		$	71,461		$	61,984


	Net income attributable to partners	$	16,345			$	6,691		$	8,068		$	12,480

	Less: net income attributable to general partner, including IDRs	490				134			161			250

	Net income attributable to limited partners	$	15,855			$	6,557		$	7,907		$	12,230


	Earnings per limited partner unit:
	Common unit – basic	$	0.3			$	0.12		$	0.16		$	0.25

	Common unit – diluted	$	0.29			$	0.12		$	0.16		$	0.25

	Subordinated unit – basic and diluted	$	0.3			$	0.12		$	0.16		$	0.25

	__________
	(1) Retrospectively adjusted for the impact of the Red Rock Drop Down and the Bison Drop Down.
	The amounts for total revenues as originally filed on the respective 2013 quarterly reports on Form 10-Q have been retrospectively adjusted for the impact of the Red Rock Drop Down and Bison Drop Down. There was no impact on net income attributable to partners or EPU. A reconciliation of total revenues follows.


		Quarter ended				Quarter ended			Quarter ended			Quarter ended
		31-Dec-13				30-Sep-13			June 30,			March 31,
									2013			2013
		(In thousands)
	Total revenues as originally reported	$	69,298			$	63,096		$	59,285		$	43,595

	Total revenue impact of Red Rock Drop Down	14,157				12,923			12,176			10,858

	Total revenue impact of Bison Drop Down	—				—			—			7,531

Total revenues	$	83,455			$	76,019		$	71,461		$	61,984

SUMMARY_OF_SIGNIFICANT_ACCOUNT1

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

Basis of Presentation

Basis of Presentation and Principles of Consolidation. We prepare our consolidated financial statements in accordance with accounting principles generally accepted in the United States of America ("GAAP"). These principles are established by the Financial Accounting Standards Board. We make estimates and assumptions that affect the reported amounts of assets and liabilities at the balance sheet dates, including fair value measurements, the reported amounts of revenue and expense, and the disclosure of contingencies. Although management believes these estimates are reasonable, actual results could differ from its estimates.

Segment Reporting

We conduct our operations in the midstream sector through four reportable segments:

•

the Marcellus Shale, which is served by Mountaineer Midstream;

•

the Williston Basin, which is served by Bison Midstream;

•

the Barnett Shale, which is served by DFW Midstream; and

•

the Piceance Basin, which is served by Grand River. Grand River is composed of the Legacy Grand River and Red Rock Gathering systems.

The assets of our reportable segments consist of natural gas gathering and processing systems and related plant and equipment. Our reportable segments reflect the way in which we internally report the financial information used to make decisions and allocate resources in connection with our operations.

Principles of Consolidation

For the purposes of the consolidated financial statements, SMLP's results of operations reflect the Partnership's operations subsequent to the IPO and the results of the Predecessor for the period prior to the IPO. The consolidated financial statements also reflect the results of operations of: (i) Red Rock Gathering since October 23, 2012, (ii) Bison Midstream since February 16, 2013 and (iii) Mountaineer Midstream since June 22, 2013. SMLP recognized its acquisitions of Red Rock Gathering and Bison Midstream at Summit Investments' historical cost because the acquisitions were executed by entities under common control. The excess of the purchase price paid by SMLP over Summit Investments' net investment in Red Rock Gathering was recognized as a reduction to partners' capital. The excess of Summit Investments' net investment in Bison Midstream over the purchase price paid by SMLP was recognized as an addition to partners' capital. Due to the common control aspect, the Red Rock Drop Down and the Bison Drop Down were accounted for by the Partnership on an “as-if pooled” basis for the periods during which common control existed. The consolidated financial statements include the assets, liabilities, and results of operations of SMLP or the Predecessor and their respective wholly owned subsidiaries. All intercompany transactions among the consolidated entities have been eliminated in consolidation.

Cash and Cash Equivalents

Cash and Cash Equivalents. Cash and cash equivalents include temporary cash investments with original maturities of three months or less.

Accounts Receivable

Prepaid Expenses

Other current assets primarily consist of the current portion of prepaid expenses that are charged to expense over the period of benefit or the life of the related contract.

Property, Plant and Equipment

Upon sale, retirement or other disposal, we remove the carrying value of an asset and its accumulated depreciation from our balance sheet and recognize the related gain or loss, if any.

Accrued capital expenditures are reflected in trade accounts payable.

Asset Retirement Obligations

Intangible Assets and Noncurrent Liabilities

Impairment of Long-Lived Assets

Goodwill

Deferred loan costs

Other noncurrent assets primarily consist of external costs incurred in connection with the issuance of our senior notes and the closing of our revolving credit facility and related amendments. We capitalize and then amortize these deferred loan costs over the life of the respective debt instrument. We recognize amortization of deferred loan costs in interest expense.

Derivatives

Fair Value of Financial Instruments

•

Level 1. Inputs represent quoted prices in active markets for identical assets or liabilities;

•

The carrying amount of cash and cash equivalents, accounts receivable, and accounts payable reported on the balance sheet approximates fair value due to their short-term maturities.

A summary of the estimated fair value of our debt financial instruments follows.

December 31, 2014

December 31, 2013

Carrying value

Estimated

Carrying value

Estimated

fair value (Level 2)

(In thousands)

Revolving credit facility

208,000

286,000

5.5% Senior notes

300,000

281,750

—

7.5% Senior notes

300,000

306,750

300,000

314,625

Nonfinancial assets and liabilities initially measured at fair value include those acquired and assumed in connection with third-party business combinations.

Commitments and Contingencies

Revenue Recognition

We obtain access to natural gas and provide services principally under contracts that contain one or more of the following arrangements:

•

Unit-Based Compensation

Income Taxes

Earnings per Unit

Comprehensive Income

Comprehensive Income. Comprehensive income is the same as net income or loss for all periods pre

Environmental Matters

Recent Accounting Pronouncements

SUMMARY_OF_SIGNIFICANT_ACCOUNT2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Estimated Fair Value for Financial instruments	A summary of the estimated fair value of our debt financial instruments follows.


		December 31, 2014				December 31, 2013
		Carrying value		Estimated		Carrying value		Estimated
				fair value (Level 2)				fair value (Level 2)
		(In thousands)
	Revolving credit facility	$	208,000	$	208,000	$	286,000	$	286,000

	5.5% Senior notes	300,000		281,750		—		—

	7.5% Senior notes	300,000		306,750		300,000		314,625

SEGMENT_INFORMATION_Tables

SEGMENT INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Schedule of assets by reportable segment	The following table presents assets by reportable segment as of December 31.


		December 31,
		2014				2013
		(In thousands)
	Assets:
	Marcellus Shale	$	243,884			$	214,379

	Williston Basin	311,041				337,610

	Barnett Shale	428,935				431,578

	Piceance Basin	872,437				876,969

	Total reportable segment assets	1,856,297				1,860,536

	Corporate	38,577				23,203

	Total assets	$	1,894,874			$	1,883,739

Schedule of segment reporting information	The following table summarizes details by reportable segment for the years ended December 31.


		Year ended December 31,
		2014				2013			2012
		(In thousands)
	Revenues:
	Marcellus Shale	$	22,694			$	9,588

	Williston Basin	62,454				50,735

	Barnett Shale	93,001				105,324			$	93,453

	Piceance Basin	152,537				127,273			81,961

	Total reportable segments	330,686				292,920			175,414

	Corporate	—				—			(991		)

	Total revenues	$	330,686			$	292,920		$	174,423


	Depreciation and amortization:
	Marcellus Shale	$	7,648			$	3,998

	Williston Basin	18,132				16,057

	Barnett Shale	15,657				13,929			$	12,078

	Piceance Basin	40,965				35,527			24,310

	Total reportable segments	82,402				69,511			36,388

	Corporate	588				451			286

	Total depreciation and amortization	$	82,990			$	69,962		$	36,674


	Other income:
	Marcellus Shale	$	—			$	—

	Williston Basin	—				—

	Barnett Shale	—				—			$	—

	Piceance Basin	1,185				—			—

	Total reportable segments	1,185				—			—

	Corporate	4				5			9

	Total other income	$	1,189			$	5		$	9


	Capital expenditures:
	Marcellus Shale	$	33,866			$	1,822

	Williston Basin	46,927				26,381

	Barnett Shale	14,567				29,534			$	39,588

	Piceance Basin	32,505				50,709			36,899

	Total reportable segments	127,865				108,446			76,487

Corporate	460				930			809

Total capital expenditures	$	128,325			$	109,376		$	77,296

The following table presents segment adjusted EBITDA by reportable segment.


	Year ended December 31,
	2014				2013			2012
	(In thousands)
Reportable segment adjusted EBITDA:
Marcellus Shale	$	15,940			$	6,333

Williston Basin	20,422				16,865

Barnett Shale	60,528				69,473			$	63,670

Piceance Basin	107,953				80,941			53,179

Total reportable segment adjusted EBITDA	$	204,843			$	173,612		$	116,849

Reconciliation of net income to adjusted EBITDA	The following table presents a reconciliation of income before income taxes to total reportable segment adjusted EBITDA.


		Year ended December 31,
		2014				2013			2012
		(In thousands)
	Reconciliation of Income Before Income Taxes to Segment Adjusted EBITDA:
	Income before income taxes	$	(20,533	)		$	54,033		$	43,679

	Add:
	Interest expense and affiliated interest expense	40,159				19,173			12,766

	Depreciation and amortization	82,990				69,962			36,674

	Amortization of favorable and unfavorable contracts	944				1,032			192

	Allocated corporate expenses	11,065				8,773			10,903

	Adjustments related to MVC shortfall payments	26,565				17,025			10,768

	Unit-based compensation	4,696				3,506			1,876

	Loss on asset sales, net	442				113			—

	Goodwill impairment	54,199				—			—

	Long-lived asset impairment	5,505				—			—

	Less:
	Interest income	4				5			9

	Impact of purchase price adjustments	1,185				—			—

	Total reportable segment adjusted EBITDA	$	204,843			$	173,612		$	116,849

PROPERTY_PLANT_AND_EQUIPMENT_N1

PROPERTY, PLANT, AND EQUIPMENT, NET (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule of property, plant, and equipment, net	Details on property, plant, and equipment, net were as follows:


		Useful lives (In years)		December 31,
				2014				2013
				(Dollars in thousands)
	Natural gas gathering and processing systems	30		$	881,235			$	744,359

	Compressor stations and compression equipment	30		410,906				380,000

	Construction in progress	n/a		33,177				83,765

	Other	15-Apr		27,345				21,304

	Total			1,352,663				1,229,428

	Less: accumulated depreciation			117,011				71,347

	Property, plant, and equipment, net			$	1,235,652			$	1,158,081

Schedule of depreciation expense related to property, plant and equipment and capitalized interest	Depreciation expense related to property, plant, and equipment and capitalized interest were as follows:


		Year ended December 31,
		2014				2013				2012
		(In thousands)
	Depreciation expense	$	45,734			$	36,745			$	22,422

	Capitalized interest	3,172				4,705				2,784

IDENTIFIABLE_INTANGIBLE_ASSETS1

IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of identifiable intangible assets and a noncurrent liability, which are subject to amortization	Identifiable intangible assets and the noncurrent liability, which are subject to amortization, were as follows:


		December 31, 2014
		Useful lives		Gross carrying amount				Accumulated amortization				Net
		(In years)
		(Dollars in thousands)
	Favorable gas gathering contracts	18.7		$	24,195			$	(8,056	)		$	16,139

	Contract intangibles	12.5		426,464				(75,713		)		350,751

	Rights-of-way	24.2		112,393				(12,417		)		99,976

	Total amortizable intangible assets			$	563,052			$	(96,186	)		$	466,866


	Unfavorable gas gathering contract	10		$	10,962			$	(5,385	)		$	5,577



		December 31, 2013
		Useful lives		Gross carrying amount				Accumulated amortization				Net
		(In years)
		(Dollars in thousands)
	Favorable gas gathering contracts	18.7		$	24,195			$	(6,315	)		$	17,880

	Contract intangibles	12.5		426,464				(43,158		)		383,306

	Rights-of-way	24.3		108,706				(7,715		)		100,991

	Total amortizable intangible assets			$	559,365			$	(57,188	)		$	502,177


	Unfavorable gas gathering contract	10		$	10,962			$	(4,588	)		$	6,374

Schedule of recognized amortization expense within the statements of operations	We recognized amortization expense in revenues as follows:


		Year ended December 31,
		2014				2013				2012
		(In thousands)
	Amortization expense – favorable gas gathering contracts	$	(1,741	)		$	(2,078	)		$	(1,715	)
	Amortization expense – unfavorable gas gathering contract	797				1,046				1,524

	Amortization of favorable and unfavorable contracts	$	(944	)		$	(1,032	)		$	(191	)
	During the fourth quarter of 2014, prices for natural gas and crude oil continued to decline such that we identified a need to evaluate the goodwill associated with the Bison Midstream system, as discussed below. In connection with this evaluation, we also evaluated the intangible assets and property, plant and equipment associated with the Bison Midstream system for impairment and concluded that no impairment was necessary.
	We recognized amortization expense in costs and expenses as follows:


		Year ended December 31,
		2014				2013				2012
		(In thousands)
	Amortization expense – contract intangibles	$	32,554			$	28,654			$	12,642

	Amortization expense – rights-of-way	4,702				4,563				1,610

Schedule of estimated aggregate amortization of intangible assets and the noncurrent liability for each of the five succeeding fiscal years	The estimated aggregate annual amortization of intangible assets and noncurrent liability expected to be recognized as of December 31, 2014 for each of the five succeeding fiscal years follows.


		Assets				Liability
		(In thousands)
	2015	$	41,882			$	698

	2016	41,846				924

	2017	40,697				1,047

	2018	40,301				1,123

	2019	40,247				957

Schedule of goodwill by segment	A rollforward of goodwill by reportable segment and in total follows.


		Piceance Basin				Williston Basin				Marcellus Shale				Total
		(In thousands)
	Goodwill, December 31, 2012	$	45,478			$	—			$	—			$	45,478

	Goodwill recognized in connection with the Bison Drop Down	—				54,199				—				54,199

	Goodwill preliminarily recognized in connection with the Mountaineer Acquisition	—				—				18,089				18,089

	Goodwill adjustment recognized in connection with finalizing accounting for the Mountaineer Acquisition and other	—				—				(1,878		)		(1,878		)

	Goodwill, December 31, 2013	45,478				54,199				16,211				115,888

	Goodwill impairment (1)	—				(54,199		)		—				(54,199		)

	Goodwill, December 31, 2014	$	45,478			$	—			$	16,211			$	61,689

DEFERRED_REVENUE_Tables

DEFERRED REVENUE (Tables)	12 Months Ended
	Dec. 31, 2014
Deferred Revenue Disclosure [Abstract]
Rollforward of deferred revenue	A rollforward of current deferred revenue follows.


		Williston Basin		Barnett		Piceance		Total
				Shale		Basin		current
		(In thousands)
	Current deferred revenue, January 1, 2012	$	—	$	—	$	—	$	—

	Additions	—		865		—		865

	Current deferred revenue, December 31, 2012	—		865		—		865

	Additions	—		1,555		—		1,555

	Less: revenue recognized due to expiration	—		865		—		865

	Current deferred revenue, December 31, 2013	—		1,555		—		1,555

	Additions	—		2,610		—		2,610

	Less: revenue recognized due to expiration	—		1,555		—		1,555

	Less: revenue recognized due to usage	—		233		—		233

	Current deferred revenue, December 31, 2014	$	—	$	2,377	$	—	$	2,377

	A rollforward of noncurrent deferred revenue follows.


		Williston Basin		Barnett		Piceance		Total noncurrent
				Shale		Basin
		(In thousands)
	Noncurrent deferred revenue, January 1, 2012	$	—	$	—	$	1,770	$	1,770

	Additions	—		—		9,129		9,129

	Noncurrent deferred revenue, December 31, 2012	—		—		10,899		10,899

	Additions(1)	6,389		—		12,395		18,784

	Noncurrent deferred revenue, December 31, 2013	6,389		—		23,294		29,683

	Additions	10,743		—		14,813		25,556

	Noncurrent deferred revenue, December 31, 2014	$	17,132	$	—	$	38,107	$	55,239

	__________
	(1) Noncurrent includes amounts recognized in connection with the Bison Drop Down.

LONGTERM_DEBT_Tables

LONG-TERM DEBT (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of long-term debt instruments	Long-term debt consisted of the following:


		December 31,
		2014		2013
		(In thousands)
	Variable rate senior secured revolving credit facility (2.67% at December 31, 2014 and 2.42% at December 31, 2013) due November 2018	$	208,000	$	286,000

	5.50% Senior unsecured notes due August 2022	300,000		—

	7.50% Senior unsecured notes due July 2021	300,000		300,000

	Total long-term debt	$	808,000	$	586,000

Schedule of maturities of long-term debt	The aggregate amount of our debt maturities during each of the years after December 31, 2014 are as follows:


		Long-term debt
		(In thousands)
	2015	$	—

	2016	—

	2017	—

	2018	208,000

	2019	—

	Thereafter	600,000

	Total long-term debt	$	808,000

PARTNERS_CAPITAL_AND_MEMBERHIP1

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS (Tables)	12 Months Ended
PARTNERS' CAPITAL AND MEMBERHIP INTERESTS (Tables)	Dec. 31, 2014
Limited Partners' Capital Account [Line Items]
Schedule of Partner Units Activity	A rollforward of the number of common limited partner, subordinated limited partner and general partner units follows.


		Common			Subordinated			General partner			Total
	Units, January 1, 2012	—			—			—			—
	Units, January 1, 2012	—			—			—			—
	Units issued to the public in connection with the IPO	14,380,000			—			—			14,380,000
	Units issued to the public in connection with the IPO	14,380,000			—			—			14,380,000
	Units issued to Summit Investments in connection with the IPO	10,029,850			24,409,850			996,320			35,436,020
		10,029,850			24,409,850			996,320			35,436,020
	Units issued under SMLP LTIP	2,577			—			—			2,577
	Units issued under SMLP LTIP	2,577			—			—			2,577
	Units, December 31, 2012	24,412,427			24,409,850			996,320			49,818,597
	Units, December 31, 2012	24,412,427			24,409,850			996,320			49,818,597
	Units issued to a subsidiary of Summit Investments in connection with the Bison Drop Down (1)	1,553,849			—			31,711			1,585,560
		1,553,849			—			31,711			1,585,560
	Units issued to a subsidiary of Summit Investments in connection with the Mountaineer Acquisition (1)	3,107,698			—			63,422			3,171,120
		3,107,698			—			63,422			3,171,120
	Units issued under SMLP LTIP	5,892			—			—			5,892
	Units issued under SMLP LTIP	5,892			—			—			5,892
	Units, December 31, 2013	29,079,866			24,409,850			1,091,453			54,581,169
	Units, December 31, 2013	29,079,866			24,409,850			1,091,453			54,581,169
	Units issued in connection with the March Equity 2014 Offering (1)	5,300,000			—			108,337			5,408,337
		5,300,000			—			108,337			5,408,337
	Units issued under SMLP LTIP (1)(2)	46,647			—			861			47,508
	Units issued under SMLP LTIP (1)(2)	46,647			—			861			47,508
	Units, December 31, 2014	34,426,513			24,409,850			1,200,651			60,037,014
	Units, December 31, 2014	34,426,513			24,409,850			1,200,651			60,037,014
	__________
	(1) Including issuance to general partner in connection with contributions made to maintain 2% general partner interest.
	(2) Units issued under SMLP LTIP in 2014 is net of 14,300 units withheld to meet minimum statutory tax withholding requirements.
Schedule of partnership target distributions

		Total quarterly distribution per unit target amount		Marginal percentage interest in distributions
		Total quarterly distribution per unit target amount		Unitholders		General partner
	Minimum quarterly distribution	$0.40		98.00%		2.00%
	First target distribution	$0.40 up to $0.46		98.00%		2.00%
	Second target distribution	above $0.46 up to $0.50		85.00%		15.00%
	Third target distribution	above $0.50 up to $0.60		75.00%		25.00%
	Thereafter	above $0.60		50.00%		50.00%
Details of Cash Distributions	Details of cash distributions declared to date follow.


	Attributable to the		Payment date		Per-unit distribution				Cash paid to common unitholders			Cash paid to subordinated unitholders		Cash paid to general partner		Cash paid for IDRs		Total distribution
	quarter ended		Payment date		Per-unit distribution				Cash paid to common unitholders			Cash paid to subordinated unitholders		Cash paid to general partner		Cash paid for IDRs		Total distribution
					(In thousands, except per-unit amounts)
	December 31, 2012		February 14, 2013		$	0.41			$	10,009		$	10,008	$	408	$	—	$	20,425
	December 31, 2012		February 14, 2013		$	0.41			$	10,009		$	10,008	$	408	$	—	$	20,425
	March 31, 2013		May 15, 2013		0.42				10,253			10,252		418		—		20,923
	March 31, 2013		May 15, 2013		0.42				10,253			10,252		418		—		20,923
	June 30, 2013		August 14, 2013		0.435				12,647			10,618		475		—		23,740
	June 30, 2013		August 14, 2013		0.435				12,647			10,618		475		—		23,740
	September 30, 2013		November 14, 2013		0.46				13,377			11,229		502		—		25,108
	September 30, 2013		November 14, 2013		0.46				13,377			11,229		502		—		25,108
	December 31, 2013		February 14, 2014		0.48				13,958			11,717		528		163		26,366
	December 31, 2013		February 14, 2014		0.48				13,958			11,717		528		163		26,366
	March 31, 2014		May 15, 2014		0.5				17,211			12,205		607		360		30,383
	March 31, 2014		May 15, 2014		0.5				17,211			12,205		607		360		30,383
	June 30, 2014		August 14, 2014		0.52				17,900			12,693		639		721		31,953
	June 30, 2014		August 14, 2014		0.52				17,900			12,693		639		721		31,953
	September 30, 2014		November 14, 2014		0.54				18,589			13,181		670		1,082		33,522
	September 30, 2014		November 14, 2014		0.54				18,589			13,181		670		1,082		33,522
Red Rock Gathering Company, LLC
Limited Partners' Capital Account [Line Items]
Calculation of capital distribution and its allocation to partners' capital	The calculation of the capital distribution and its allocation to partners' capital follow (in thousands).


	Summit Investments' net investment in Red Rock Gathering					$	241,817
	Summit Investments' net investment in Red Rock Gathering					$	241,817
	Total cash consideration paid to a subsidiary of Summit Investments					307,941
						307,941
	Excess of purchase price over acquired carrying value of Red Rock Gathering					$	(66,124	)

	Allocation of capital distribution:
	General partner interest	$	(1,323	)
	Common limited partner interest	(37,910		)
	Subordinated limited partner interest	(26,891		)
	Partners' capital allocation					$	(66,124	)
Bison Midstream
Limited Partners' Capital Account [Line Items]
Calculation of capital contribution and allocation to partners' capital	The details of total cash and unit consideration as well as the calculation of the capital contribution and its allocation to partners' capital follow (dollars in thousands).


	Summit Investments' net investment in Bison Midstream					$	305,449
	Summit Investments' net investment in Bison Midstream					$	305,449
	Aggregate cash paid to Summit Investments	$	200,000
	Aggregate cash paid to Summit Investments	$	200,000
	Issuance of 1,553,849 SMLP common units to Summit Investments	47,936
		47,936
	Issuance of 31,711 SMLP general partner units to the general partner	978
		978
	Total consideration					248,914
	Total consideration					248,914
	Summit Investments' contribution of net assets in excess of consideration					$	56,535
						$	56,535

	Allocation of capital contribution:
	General partner interest	$	1,131
	General partner interest	$	1,131
	Common limited partner interest	28,558
	Common limited partner interest	28,558
	Subordinated limited partner interest	26,846
	Subordinated limited partner interest	26,846
	Partners' capital allocation					$	56,535
	Partners' capital allocation					$	56,535
Mountaineer Midstream
Limited Partners' Capital Account [Line Items]
Calculation of capital contribution and allocation to partners' capital	The allocation and valuation of units issued to partially fund the Mountaineer Acquisition follow (dollars in thousands).


	Issuance of 3,107,698 SMLP common units to Summit Investments	$	98,000
		$	98,000
	Issuance of 63,422 SMLP general partner units to the general partner	2,000
		2,000
	Issuance of units in connection with the Mountaineer Acquisition	$	100,000
		$	100,000

EARNINGS_PER_UNIT_Tables

EARNINGS PER UNIT (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule of earnings per Unit	The following table presents details on EPU.


		Year ended December 31,
		2014			2013		2012 (1)
		(In thousands, except per-unit amounts)
	Net (loss) income	$	(21,164	)	$	53,304	$	42,997

	Less: net income attributable to the pre-IPO period	—			—		24,112

	Less: net income attributable to Summit Investments	2,828			9,720		1,271

	Net (loss) income attributable to SMLP	(23,992		)	43,584		17,614

	Less: net (loss) income attributable to general partner, including IDRs	3,125			1,035		352

	Net (loss) income attributable to limited partners	$	(27,117	)	$	42,549	$	17,262


	Numerator for basic and diluted EPU:
	Allocation of net (loss) income among limited partner interests:
	Net (loss) income attributable to common units	$	(16,324	)	$	23,227	$	8,632

	Net (loss) income attributable to subordinated units	(10,793		)	19,322		8,630

	Net (loss) income attributable to limited partners	$	(27,117	)	$	42,549	$	17,262


	Denominator for basic and diluted EPU:
	Weighted-average common units outstanding – basic	33,311			26,951		24,412

	Effect of nonvested phantom units	—			150		132

	Weighted-average common units outstanding – diluted	33,311			27,101		24,544


	Weighted-average subordinated units outstanding – basic and diluted	24,410			24,410		24,410


	(Loss) earnings per limited partner unit:
	Common unit – basic	$	(0.49	)	$	0.86	$	0.35

	Common unit – diluted	$	(0.49	)	$	0.86	$	0.35

	Subordinated unit – basic and diluted	$	(0.44	)	$	0.79	$	0.35

	__________
	(1) Calculated for the period from October 1, 2012 to December 31, 2012

UNITBASED_COMPENSATION_Tables

UNIT-BASED COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2014
Phantom and Restricted units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of activity	The following table presents phantom and restricted unit activity:


		Units		Weighted-average grant date
				fair value
	Nonvested phantom and restricted units, January 1, 2012	—		$	—

	Phantom units granted	125,000		20

	Restricted units granted	6,558		20.23

	Nonvested phantom and restricted units, December 31, 2012	131,558		20

	Phantom units granted	155,330		26.33

	Restricted units granted	835		27.5

	Phantom units forfeited	(4,041	)	25.99

	Nonvested phantom and restricted units, December 31, 2013	283,682		23.41

	Phantom units granted	136,867		42.32

	Phantom and restricted units vested	(61,917	)	25.33

	Phantom units forfeited	(22,430	)	25.56

	Nonvested phantom units, December 31, 2014	336,202		$	30.61

DFW Net Profit Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of activity	Information regarding the vested and nonvested DFW Net Profits Interests were as follows:


		Year ended December 31,
		2013					2012
		Percentage Interest		Weighted-average grant date fair value (per 1.0% of DFW Net Profits Interest)			Percentage Interest			Weighted-average grant date fair value (per 1.0% of DFW Net Profits Interest)
		(Dollars in thousands)
	Nonvested, beginning of period	0.038	%	$	1,650		1.75	%		$	306

	Repurchased	0.038	%	$	1,650		0	%		$	—

	Vested	0	%	$	—		1.644	%		$	256

	Forfeited	0	%	$	—		0.069	%		$	765

	Nonvested, end of period	0	%	$	—		0.038	%		$	1,650


	Vested, end of period	0	%	$	—		4.294	%		$	257

SMP Net Profits Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of inputs used in the option pricing method	The inputs used in the option pricing method for the SMP Net Profits Interests granted during the year ended December 31, 2012 were as follows:


		January
		2012
		grant
	Length of holding period restriction (In years)	2.93

	Discount for lack of marketability	24	%
	Volatility	37	%
Schedule of activity	Information regarding the amount and grant-date fair value of the vested and nonvested SMP Net Profits Interests for the period in which they were reflected in our financial results follows.


		Year ended December 31,
		2012
		Percentage Interest		Weighted-average grant date fair value (per 1.0% of SMP Net Profits Interest)
		(Dollars in thousands)
	Nonvested, beginning of period	3.958	%	$	1,003

	Granted	0.5	%	$	1,780

	Vested	1.271	%	$	965

	Nonvested, end of period (1)	3.187	%	$	1,140


	Vested, end of period	3.168	%	$	788

	__________
	(1) Subsequent to the IPO, the vested and nonvested net profits interests are obligations of the Predecessor and not the Partnership
Long-term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of non-cash compensation expense recognized in general and administrative expense	Unit-based compensation recognized in general and administrative expense related to awards under the SMLP LTIP was as follows:


		Year ended December 31,
		2014			2013			2012
		(In thousands)
	SMLP LTIP unit-based compensation	$	4,696		$	2,999		$	269

CONCENTRATIONS_OF_RISK_Tables

CONCENTRATIONS OF RISK (Tables)	12 Months Ended
	Dec. 31, 2014
Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Schedule of customers accounting for more than 10% of total revenues and accounts receivable	Counterparties accounting for more than 10% of total revenues were as follows:


		Year ended December 31,
		2014		2013		2012
	Revenue:
	Counterparty A - Piceance Basin	22	%	19	%	27	%
	Counterparty B - Williston Basin	10	%	*		—	%

	Counterparty C - Williston Basin	*		*		—	%

	Counterparty D - Marcellus Shale	*		*		—	%

	Counterparty E - Barnett Shale	*		15	%	19	%

	Counterparty F - Barnett Shale	*		*		14	%

	__________
	* Less than 10%
Accounts receivable \| Credit concentration
CONCENTRATIONS OF RISK
Schedule of customers accounting for more than 10% of total revenues and accounts receivable	Counterparties accounting for more than 10% of total accounts receivable were as follows:


		December 31,
		2014		2013
	Accounts receivable:
	Counterparty A - Piceance Basin	29	%	37	%
	Counterparty B - Williston Basin	*		*

	Counterparty C - Williston Basin	14	%	*

	Counterparty D - Marcellus Shale	10	%	*

	Counterparty E - Barnett Shale	*		11	%

	Counterparty F - Barnett Shale	*		*

	__________
	* Less than 10%

RELATEDPARTY_TRANSACTIONS_Tabl

RELATED-PARTY TRANSACTIONS (Tables)	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Schedules of Related Party Transactions	Amounts paid for such services were as follows:


		Year ended December 31,
		2014		2013		2012
		(In thousands)
	Payments for electricity management consulting services	$	234	$	199	$	204

	Expenses incurred by the general partner and reimbursed by us under our partnership agreement were as follows:


		Year ended December 31,
		2014		2013		2012
		(In thousands)
	Operation and maintenance expense	$	17,223	$	13,808	$	2,913

	General and administrative expense	18,843		17,362		3,661

COMMITMENTS_AND_CONTINGENCIES_

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of total rent expense related to operating leases	Rent expense related to operating leases, including rent expense incurred on our behalf and allocated to us, was as follows:


		Year ended December 31,
		2014		2013		2012

	Rent expense	$	1,677	$	1,381	$	732

ACQUISITIONS_Tables

ACQUISITIONS (Tables)	12 Months Ended
	Dec. 31, 2014
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information

		Year ended December 31,
		2014			2013			2012
		(In thousands, except for per-unit amounts)
	Total Red Rock Gathering revenues included in consolidated revenues	$	73,266		$	50,114		$	8,924

	Total Bison Midstream and Mountaineer Midstream revenues included in consolidated revenues				60,323


	Total Red Rock Gathering net income included in consolidated net income	$	27,447		$	9,668		$	1,271

	Total Bison Midstream and Mountaineer Midstream net loss included in consolidated net income				(457		)

	Pro forma total revenues	$	330,686		$	305,071		$	256,637

	Pro forma net (loss) income	(20,938		)	47,371			38,639


	Pro forma common EPU - basic and diluted	$	(0.41	)	$	0.79		$	0.28

	Pro forma subordinated EPU - basic and diluted	(0.41		)	0.79			0.28

Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Fair Value of Assets Acquired and Liabilities Assumed	The final fair values of the assets acquired and liabilities assumed as of October 23, 2012, were as follows (in thousands):


	Red Rock Gathering purchase price				$	206,694

	Cash	$	1,097

	Accounts receivable	8,018

	Other assets	317

	Property, plant, and equipment	150,401

	Rights-of-way	52,197

	Other noncurrent assets	164

	Total assets acquired	212,194

	Trade accounts payable	2,558

	Other current liabilities	2,942

	Total liabilities assumed	$	5,500

	Net identifiable assets acquired				$	206,694

Bison Midstream
Business Acquisition [Line Items]
Fair Value of Assets Acquired and Liabilities Assumed	The fair values of the assets acquired and liabilities assumed as of February 15, 2013, were as follows (in thousands):


	Purchase price assigned to Bison Gas Gathering system				$	303,168

	Current assets	$	5,705

	Property, plant, and equipment	85,477

	Intangible assets	164,502

	Other noncurrent assets	2,187

	Total assets acquired	257,871

	Current liabilities	6,112

	Other noncurrent liabilities	2,790

	Total liabilities assumed	$	8,902

	Net identifiable assets acquired				248,969

	Goodwill				$	54,199

Schedule of Revenue and Net Income Disclosures	Revenues and net income for the previously separate entities and the combined amounts for the years ended December 31, 2014, 2013 and 2012, as presented in these consolidated financial statements follow.


		Year ended December 31,
		2014			2013			2012
		(In thousands)
	SMLP revenues	$	319,373		$	225,192		$	165,499

	Red Rock Gathering revenues	11,313			50,114			8,924

	Bison Gas Gathering system revenues (1)				17,614

	Combined revenues	$	330,686		$	292,920		$	174,423


	SMLP net (loss) income	$	(23,992	)	$	43,584		$	41,726

	Red Rock Gathering net income	2,828			9,668			1,271

	Bison Gas Gathering system net income (1)				52

	Combined net (loss) income	$	(21,164	)	$	53,304		$	42,997

	__________
	(1) Results are fully reflected in SMLP's results of operations for the year ended December 31, 2014.
Mountaineer Midstream
Business Acquisition [Line Items]
Fair Value of Assets Acquired and Liabilities Assumed	The final fair values of the assets acquired and liabilities assumed as of June 21, 2013, were as follows (in thousands):


	Purchase price assigned to Mountaineer Midstream				$	210,000

	Property, plant, and equipment	$	163,661

	Gas gathering agreement contract intangibles	24,019

	Rights-of-way	6,109

	Total assets acquired	193,789

	Total liabilities assumed	$	—

	Net identifiable assets acquired				193,789

	Goodwill				$	16,211

UNAUDITED_QUARTERLY_FINANCIAL_1

UNAUDITED QUARTERLY FINANCIAL DATA (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly summarized information on the consolidated results of operations	Summarized information on the consolidated results of operations for each of the quarters during the two-year period ended December 31, 2014, follows.


		Quarter ended			Quarter ended		Quarter ended		Quarter ended
		31-Dec-14			30-Sep-14		June 30,		March 31,
							2014		2014
		(In thousands, except per-unit amounts)
	Total revenues	$	94,658		$	79,030	$	80,796	$	76,202


	Net (loss) income attributable to partners(1)	$	(37,686	)	$	6,113	$	4,036	$	3,545

	Less: net (loss) income attributable to general partner, including IDRs	689			1,204		801		431

	Net (loss) income attributable to limited partners	$	(38,375	)	$	4,909	$	3,235	$	3,114


	(Loss) earnings per limited partner unit:
	Common unit – basic	$	(0.65	)	$	0.08	$	0.05	$	0.08

	Common unit – diluted	$	(0.65	)	$	0.08	$	0.05	$	0.08

	Subordinated unit – basic and diluted	$	(0.65	)	$	0.08	$	0.05	$	0.02

	__________
	(1) In the quarter ended December 31, 2014, includes $54.2 million of goodwill impairment and $5.5 million of long-lived asset impairment.


		Quarter ended			Quarter ended		Quarter ended		Quarter ended
		31-Dec-13			30-Sep-13		June 30,		March 31,
							2013		2013
		(In thousands, except per-unit amounts)
	Total revenues(1)	$	83,455		$	76,019	$	71,461	$	61,984


	Net income attributable to partners	$	16,345		$	6,691	$	8,068	$	12,480

	Less: net income attributable to general partner, including IDRs	490			134		161		250

	Net income attributable to limited partners	$	15,855		$	6,557	$	7,907	$	12,230


	Earnings per limited partner unit:
	Common unit – basic	$	0.3		$	0.12	$	0.16	$	0.25

	Common unit – diluted	$	0.29		$	0.12	$	0.16	$	0.25

	Subordinated unit – basic and diluted	$	0.3		$	0.12	$	0.16	$	0.25

	__________
	(1) Retrospectively adjusted for the impact of the Red Rock Drop Down and the Bison Drop Down.
Retrospective adjustments from impact of acquisitions	There was no impact on net income attributable to partners or EPU. A reconciliation of total revenues follows.


		Quarter ended			Quarter ended		Quarter ended		Quarter ended
		31-Dec-13			30-Sep-13		June 30,		March 31,
							2013		2013
		(In thousands)
	Total revenues as originally reported	$	69,298		$	63,096	$	59,285	$	43,595

	Total revenue impact of Red Rock Drop Down	14,157			12,923		12,176		10,858

	Total revenue impact of Bison Drop Down	—			—		—		7,531

	Total revenues	$	83,455		$	76,019	$	71,461	$	61,984

ORGANIZATION_BUSINESS_OPERATIO1

ORGANIZATION, BUSINESS OPERATIONS AND BASIS OF PRESENTATION (Details)	1 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended
	Mar. 31, 2013	Dec. 31, 2014	Oct. 03, 2012	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014
	Mar. 31, 2013	segment	Oct. 03, 2012	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014
ORGANIZATION AND BUSINESS OPERATIONS
Limited partner interest (as a percent)	100.00%
Common limited partner capital, units outstanding		34,427,000			29,080,000
Number of reportable segments		4
Summit Holdings
ORGANIZATION AND BUSINESS OPERATIONS
Cumulative percentage ownership after all transactions			100.00%
DFW Midstream
ORGANIZATION AND BUSINESS OPERATIONS
Cumulative percentage ownership after all transactions			100.00%
Grand River Gathering, LLC
ORGANIZATION AND BUSINESS OPERATIONS
Cumulative percentage ownership after all transactions			100.00%
Common units \| Summit Investments
ORGANIZATION AND BUSINESS OPERATIONS
Common limited partner capital, units outstanding		5,293,571
General partner
ORGANIZATION AND BUSINESS OPERATIONS
General partner interest (as a percent)	2.00%	2.00%	2.00%	2.00%	2.00%	2.00%
Class A Membership Interest \| GE Energy Financial Services
ORGANIZATION AND BUSINESS OPERATIONS
Exchange of membership interests, percent							100.00%

ORGANIZATION_BUSINESS_OPERATIO2

ORGANIZATION, BUSINESS OPERATIONS AND BASIS OF PRESENTATION - Initial Public Offering (Details) (USD $)	0 Months Ended	1 Months Ended		12 Months Ended
	Oct. 03, 2012	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Limited Partners' Capital Account [Line Items]
Limited partner interest (as a percent)			100.00%
Distributions made to reimburse for certain capital expenditures	88,000,000
Common units sold in public offering				5,408,337		14,380,000
Net proceeds from IPO	269,400,000			197,806,000	0	263,125,000
Repayments of indebtedness				315,295,000	294,180,000	160,770,000
Distributions made in connection with underwriters exercising option to purchase additional units	35,100,000
Estimated IPO expenses	6,300,000
General partner
Limited Partners' Capital Account [Line Items]
General partner interest (as a percent)	2.00%	2.00%	2.00%	2.00%	2.00%	2.00%
Partner units	996,320
Revolving credit facility
Limited Partners' Capital Account [Line Items]
Repayments of indebtedness	140,000,000
Common units \| Limited Partner
Limited Partners' Capital Account [Line Items]
Partner units	10,029,850
Units sold to public for cash in connection with the exercise of the underwriters' option to purchase additional common units	1,875,000
Limited partner interest (as a percent)	20.10%
Common units sold in public offering	14,375,000
Limited partner interest sold in public offering (as a percent)	28.90%
Subordinated Units \| Limited Partner
Limited Partners' Capital Account [Line Items]
Partner units	24,409,850
Limited partner interest (as a percent)	49.00%
Director \| Common units
Limited Partners' Capital Account [Line Items]
Units granted in connection with IPO pursuant to LTIP	5,000
Number of related persons to whom common units were granted	2
Phantom Units
Limited Partners' Capital Account [Line Items]
Units granted in connection with IPO pursuant to LTIP				136,867	155,330	125,000
Phantom Units \| Employees
Limited Partners' Capital Account [Line Items]
Units granted in connection with IPO pursuant to LTIP	125,000

SUMMARY_OF_SIGNIFICANT_ACCOUNT3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) (USD $)	12 Months Ended		36 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts	$0	$0	$0
Unfavorable gas gathering contract, useful lives	10 years	10 years
Impairment of long-lived assets			$0
Favorable gas gathering contract
Significant Accounting Policies [Line Items]
Useful lives	18 years 8 months 12 days	18 years 8 months 12 days
Rights-of-way
Significant Accounting Policies [Line Items]
Useful lives	24 years 2 months	24 years 4 months
Gathering System
Significant Accounting Policies [Line Items]
Useful lives	30 years
Minimum
Significant Accounting Policies [Line Items]
Unfavorable gas gathering contract, useful lives	10 years
Minimum \| Favorable gas gathering contract
Significant Accounting Policies [Line Items]
Useful lives	10 years
Minimum \| Other Gas Gathering Contract
Significant Accounting Policies [Line Items]
Useful lives	10 years
Minimum \| Rights-of-way
Significant Accounting Policies [Line Items]
Useful lives	20 years
Maximum
Significant Accounting Policies [Line Items]
Unfavorable gas gathering contract, useful lives	20 years
Maximum \| Favorable gas gathering contract
Significant Accounting Policies [Line Items]
Useful lives	20 years
Maximum \| Other Gas Gathering Contract
Significant Accounting Policies [Line Items]
Useful lives	25 years
Maximum \| Rights-of-way
Significant Accounting Policies [Line Items]
Useful lives	30 years

SUMMARY_OF_SIGNIFICANT_ACCOUNT4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revolving credit facility, Carrying value	$208,000,000	$286,000,000
Fair value, Level 2 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Revolving credit facility, Estimated fair value (Level 2)	208,000,000	286,000,000
5.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, Carrying value	300,000,000	0
5.5% Senior Notes \| Fair value, Level 2 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, Estimated fair value (Level 2)	281,750,000	0
7.5% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, Carrying value	300,000,000	300,000,000
7.5% Senior Notes \| Fair value, Level 2 inputs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes, Estimated fair value (Level 2)	$306,750,000	$314,625,000

SUMMARY_OF_SIGNIFICANT_ACCOUNT5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income Taxes and EPS (Details) (USD $)	0 Months Ended	1 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Oct. 03, 2012	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Significant Accounting Policies [Line Items]
Deferred tax liability due to changes from the Texas Margin Tax				1
Reduction in current income tax expense				0.3
Deferred tax liability, noncurrent				1.3
General partner
Significant Accounting Policies [Line Items]
General partner interest (as a percent)	2.00%	2.00%	2.00%	2.00%	2.00%	2.00%

SEGMENT_INFORMATION_Assets_by_

SEGMENT INFORMATION - Assets by Reportable Segment (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Assets	$1,894,874	$1,883,739
Reportable Segments
Segment Reporting Information [Line Items]
Assets	1,856,297	1,860,536
Reportable Segments \| Marcellus Shale Segment
Segment Reporting Information [Line Items]
Assets	243,884	214,379
Reportable Segments \| Williston Basin Segment
Segment Reporting Information [Line Items]
Assets	311,041	337,610
Reportable Segments \| Barnett Shale Segment
Segment Reporting Information [Line Items]
Assets	428,935	431,578
Reportable Segments \| Piceance Basin Segment
Segment Reporting Information [Line Items]
Assets	872,437	876,969
Corporate
Segment Reporting Information [Line Items]
Assets	$38,577	$23,203

SEGMENT_INFORMATION_Adjusted_E

SEGMENT INFORMATION - Adjusted EBITDA by Reportable Segment (Details) (Reportable Segments, USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Adjusted EBITDA	$204,843	$173,612	$116,849
Marcellus Shale Segment
Segment Reporting Information [Line Items]
Adjusted EBITDA	15,940	6,333
Williston Basin Segment
Segment Reporting Information [Line Items]
Adjusted EBITDA	20,422	16,865
Barnett Shale Segment
Segment Reporting Information [Line Items]
Adjusted EBITDA	60,528	69,473	63,670
Piceance Basin Segment
Segment Reporting Information [Line Items]
Adjusted EBITDA	$107,953	$80,941	$53,179

SEGMENT_INFORMATION_Reconcilia

SEGMENT INFORMATION - Reconciliation of Net Income to Adjusted EBITDA (Details) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Income before income taxes			($20,533,000)	$54,033,000	$43,679,000
Add:
Interest expense and affiliated interest expense			40,159,000	19,173,000	12,766,000
Depreciation and amortization			82,990,000	69,962,000	36,674,000
Amortization of favorable and unfavorable contracts			944,000	1,032,000	192,000
Adjustments related to MVC shortfall payments			26,565,000	17,025,000	10,768,000
Unit-based compensation			4,696,000	3,506,000	1,876,000
Loss on asset sales, net			442,000	113,000	0
Goodwill impairment	54,200,000	0	54,199,000	0	0
Long-lived asset impairment	5,500,000		5,505,000	0	0
Less:
Interest income			4,000	5,000	9,000
Impact of purchase price adjustments			1,185,000	0	0
Corporate
Add:
Depreciation and amortization			588,000	451,000	286,000
Allocated corporate expenses			11,065,000	8,773,000	10,903,000
Reportable Segments
Add:
Depreciation and amortization			82,402,000	69,511,000	36,388,000
Less:
Adjusted EBITDA			$204,843,000	$173,612,000	$116,849,000

SEGMENT_INFORMATION_Revenues_D

SEGMENT INFORMATION - Revenues, Depreciation and Amortization, Other (Expense) Income, and Capital Expenditures by Reportable Segment (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$94,658	$79,030	$80,796	$76,202	$83,455	$76,019	$71,461	$61,984	$330,686	$292,920	$174,423
Depreciation and amortization									82,990	69,962	36,674
Other income									1,189	5	9
Capital expenditures									128,325	109,376	77,296
Reportable Segments
Segment Reporting Information [Line Items]
Revenues									330,686	292,920	175,414
Depreciation and amortization									82,402	69,511	36,388
Other income									1,185	0	0
Capital expenditures									127,865	108,446	76,487
Reportable Segments \| Marcellus Shale Segment
Segment Reporting Information [Line Items]
Revenues									22,694	9,588
Depreciation and amortization									7,648	3,998
Other income									0	0
Capital expenditures									33,866	1,822
Reportable Segments \| Williston Basin Segment
Segment Reporting Information [Line Items]
Revenues									62,454	50,735
Depreciation and amortization									18,132	16,057
Other income									0	0
Capital expenditures									46,927	26,381
Reportable Segments \| Barnett Shale Segment
Segment Reporting Information [Line Items]
Revenues									93,001	105,324	93,453
Depreciation and amortization									15,657	13,929	12,078
Other income									0	0	0
Capital expenditures									14,567	29,534	39,588
Reportable Segments \| Piceance Basin Segment
Segment Reporting Information [Line Items]
Revenues									152,537	127,273	81,961
Depreciation and amortization									40,965	35,527	24,310
Other income									1,185	0	0
Capital expenditures									32,505	50,709	36,899
Corporate
Segment Reporting Information [Line Items]
Revenues									0	0	-991
Depreciation and amortization									588	451	286
Other income									4	5	9
Capital expenditures									$460	$930	$809

PROPERTY_PLANT_AND_EQUIPMENT_N2

PROPERTY, PLANT, AND EQUIPMENT, NET (Details) (USD $)	3 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Gross	$1,352,663	$1,352,663	$1,229,428
Less: accumulated depreciation	117,011	117,011	71,347
Property, plant, and equipment, net	1,235,652	1,235,652	1,158,081
Long-lived asset impairment	5,500	5,505	0	0
Depreciation expense		45,734	36,745	22,422
Capitalized interest		3,172	4,705	2,784
Gas gathering system
Property, Plant and Equipment [Line Items]
Useful lives		30 years	30 years
Gross	881,235	881,235	744,359
Compressor stations and compression equipment
Property, Plant and Equipment [Line Items]
Useful lives		30 years	30 years
Gross	410,906	410,906	380,000
Long-lived asset impairment	5,500
Construction in progress
Property, Plant and Equipment [Line Items]
Gross	33,177	33,177	83,765
Other
Property, Plant and Equipment [Line Items]
Gross	$27,345	$27,345	$21,304
Other \| Minimum
Property, Plant and Equipment [Line Items]
Useful lives		4 years	4 years
Other \| Maximum
Property, Plant and Equipment [Line Items]
Useful lives		15 years	15 years

IDENTIFIABLE_INTANGIBLE_ASSETS2

IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$563,052	$559,365
Accumulated amortization	-96,186	-57,188
Total intangible assets, net	466,866	502,177
Noncurrent liability
Unfavorable gas gathering contract, useful lives	10 years	10 years
Unfavorable contract, Gross Carrying Amount	10,962	10,962
Unfavorable gas gathering contract, Accumulated Amortization	-5,385	-4,588
Unfavorable gas gathering contract, Net	5,577	6,374
Amortization expense
Amortization expense - unfavorable contract	797	1,046	1,524
Amortization of favorable and unfavorable contracts	-944	-1,032	-191
Assets
2015	41,882
2016	41,846
2017	40,697
2018	40,301
2019	40,247
Liability
2015	698
2016	924
2017	1,047
2018	1,123
2019	957
Favorable gas gathering contract
Finite-Lived Intangible Assets [Line Items]
Useful lives	18 years 8 months 12 days	18 years 8 months 12 days
Gross carrying amount	24,195	24,195
Accumulated amortization	-8,056	-6,315
Total intangible assets, net	16,139	17,880
Amortization expense
Amortization expense	1,741	2,078	1,715
Contract intangibles
Finite-Lived Intangible Assets [Line Items]
Useful lives	12 years 6 months	12 years 6 months
Gross carrying amount	426,464	426,464
Accumulated amortization	-75,713	-43,158
Total intangible assets, net	350,751	383,306
Amortization expense
Amortization expense	32,554	28,654	12,642
Rights-of-way
Finite-Lived Intangible Assets [Line Items]
Useful lives	24 years 2 months	24 years 4 months
Gross carrying amount	112,393	108,706
Accumulated amortization	-12,417	-7,715
Total intangible assets, net	99,976	100,991
Amortization expense
Amortization expense	$4,702	$4,563	$1,610

IDENTIFIABLE_INTANGIBLE_ASSETS3

IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL - Goodwill (Details) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 15, 2013	Jun. 21, 2013
Goodwill [Roll Forward]
Goodwill, beginning balance		$115,888,000	$115,888,000	$45,478,000
Goodwill impairment	-54,200,000	0	-54,199,000	0	0
Goodwill, ending balance	61,689,000		61,689,000	115,888,000	45,478,000
Piceance Basin Segment
Goodwill [Roll Forward]
Goodwill, beginning balance		45,478,000	45,478,000
Goodwill impairment			0
Goodwill, ending balance	45,478,000		45,478,000		45,478,000
Williston Basin Segment
Goodwill [Roll Forward]
Goodwill, beginning balance		54,199,000	54,199,000
Goodwill impairment			-54,199,000
Goodwill, ending balance	0		0		0
Marcellus Shale Segment
Goodwill [Roll Forward]
Goodwill, beginning balance		16,211,000	16,211,000
Goodwill impairment			0
Goodwill, ending balance	16,211,000		16,211,000		0
Bison Midstream
Goodwill [Roll Forward]
Goodwill, beginning balance						54,199,000
Goodwill acquired				54,199,000
Goodwill, ending balance						54,199,000
Bison Midstream \| Piceance Basin Segment
Goodwill [Roll Forward]
Goodwill acquired				0
Bison Midstream \| Williston Basin Segment
Goodwill [Roll Forward]
Goodwill acquired				54,199,000
Bison Midstream \| Marcellus Shale Segment
Goodwill [Roll Forward]
Goodwill acquired				0
Mountaineer Midstream
Goodwill [Roll Forward]
Goodwill, beginning balance							16,211,000
Goodwill acquired				18,089,000
Goodwill adjustment recognized in connection with finalizing accounting for the Mountaineer Acquisition and other				-1,878,000
Goodwill, ending balance							16,211,000
Mountaineer Midstream \| Piceance Basin Segment
Goodwill [Roll Forward]
Goodwill acquired				0
Goodwill adjustment recognized in connection with finalizing accounting for the Mountaineer Acquisition and other				0
Mountaineer Midstream \| Williston Basin Segment
Goodwill [Roll Forward]
Goodwill acquired				0
Goodwill adjustment recognized in connection with finalizing accounting for the Mountaineer Acquisition and other				0
Mountaineer Midstream \| Marcellus Shale Segment
Goodwill [Roll Forward]
Goodwill acquired				18,089,000
Goodwill adjustment recognized in connection with finalizing accounting for the Mountaineer Acquisition and other				($1,878,000)

IDENTIFIABLE_INTANGIBLE_ASSETS4

IDENTIFIABLE INTANGIBLE ASSETS, NONCURRENT LIABILITY AND GOODWILL - Additional Disclosures (Details) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment	$54,200,000	$0	$54,199,000	$0	$0

DEFERRED_REVENUE_Details

DEFERRED REVENUE (Details) (USD $)	12 Months Ended
DEFERRED REVENUE (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$1,555,000	$865,000	$0
Additions	2,610,000	1,555,000	865,000
Less: revenue recognized due to expiration	1,555,000	865,000
Less: revenue recognized due to usage	233,000
Current deferred revenue, ending balance	2,377,000	1,555,000	865,000
Noncurrent deferred revenue, beginning balance	29,683,000	10,899,000	1,770,000
Additions	25,556,000	18,784,000	9,129,000
Noncurrent deferred revenue, ending balance	55,239,000	29,683,000	10,899,000
Shortfall payments billed and included in accounts receivable	13,100,000
Maximum period customers have the ability to offset their gathering fees	7 years
Williston Basin Segment
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0	0
Additions	0	0	0
Less: revenue recognized due to expiration	0	0
Less: revenue recognized due to usage	0
Current deferred revenue, ending balance	0	0	0
Noncurrent deferred revenue, beginning balance	6,389,000	0	0
Additions	10,743,000	6,389,000	0
Noncurrent deferred revenue, ending balance	17,132,000	6,389,000	0
Barnett Shale Segment
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	1,555,000	865,000	0
Additions	2,610,000	1,555,000	865,000
Less: revenue recognized due to expiration	1,555,000	865,000
Less: revenue recognized due to usage	233,000
Current deferred revenue, ending balance	2,377,000	1,555,000	865,000
Noncurrent deferred revenue, beginning balance	0	0	0
Additions	0	0	0
Noncurrent deferred revenue, ending balance	0	0	0
Piceance Basin Segment
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	0	0	0
Additions	0	0	0
Less: revenue recognized due to expiration	0	0
Less: revenue recognized due to usage	0
Current deferred revenue, ending balance	0	0	0
Noncurrent deferred revenue, beginning balance	23,294,000	10,899,000	1,770,000
Additions	14,813,000	12,395,000	9,129,000
Noncurrent deferred revenue, ending balance	$38,107,000	$23,294,000	$10,899,000

LONGTERM_DEBT_Details

LONG-TERM DEBT (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Revolving credit facility	$208,000,000	$286,000,000
Total long-term debt	808,000,000	586,000,000
Revolving credit facility
Line of Credit Facility [Line Items]
Weighted-average interest rate (as a percent)	2.67%	2.42%
5.5% Senior Notes
Line of Credit Facility [Line Items]
Senior unsecured notes	300,000,000	0
7.5% Senior Notes
Line of Credit Facility [Line Items]
Senior unsecured notes	$300,000,000	$300,000,000
Senior Notes \| 5.5% Senior Notes
Line of Credit Facility [Line Items]
Stated interest rate	5.50%	5.50%
Senior Notes \| 7.5% Senior Notes
Line of Credit Facility [Line Items]
Stated interest rate	7.50%	7.50%

LONGTERM_DEBT_Maturities_Detai

LONG-TERM DEBT - Maturities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
2015	$0
2016	0
2017	0
2018	208,000
2019	0
Thereafter	600,000
Long-term Debt	$808,000	$586,000

LONGTERM_DEBT_Revolving_Credit

LONG-TERM DEBT - Revolving Credit Facility (Details) (Revolving credit facility, USD $)	12 Months Ended
	Dec. 31, 2014
Line of Credit Facility [Line Items]
Borrowing capacity	$700,000,000
Commitment fee on unused portion of the facility (as a percent)	0.50%
Interest rate at period end	2.67%
Unused portion under the facility	492,000,000
Maximum cumulative lease payment obligations allowable under terms of covenants	30,000,000
Period that cumulative lease payment obligations may not exceed specified amount under terms of covenants	12 months
Trailing period used in calculating the ratio of EBITDA to net interest expense	12 months
Trailing period used in calculating the ratio of total indebtedness to consolidated EBITDA	12 months
Period following certain acquisitions, for which higher ratio of total indebtedness to consolidated EBITDA is to be maintained	270 days
Debt defaults	0
LIBOR
Line of Credit Facility [Line Items]
Applicable margin (as a percent)	2.50%
Accordion provision
Line of Credit Facility [Line Items]
Borrowing capacity	$200,000,000
Minimum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility (as a percent)	0.30%
Ratio of consolidated EBITDA to net interest expense	2.5
Minimum \| ABR
Line of Credit Facility [Line Items]
Applicable margin (as a percent)	0.75%
Minimum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin (as a percent)	1.75%
Maximum
Line of Credit Facility [Line Items]
Commitment fee on unused portion of the facility (as a percent)	0.50%
Ratio of total indebtedness to consolidated EBITDA	5
Ratio of total indebtedness to consolidated EBITDA, for a specified period following certain acquisitions	5.5
Maximum \| ABR
Line of Credit Facility [Line Items]
Applicable margin (as a percent)	1.75%
Maximum \| LIBOR
Line of Credit Facility [Line Items]
Applicable margin (as a percent)	2.75%

LONGTERM_DEBT_Senior_Notes_Det

LONG-TERM DEBT - Senior Notes (Details) (USD $)	0 Months Ended	12 Months Ended
LONG-TERM DEBT - Senior Notes (Details) (USD $)	Jul. 15, 2014	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2013
Finance Corp.
Debt Instrument [Line Items]
Cumulative percentage ownership in subsidiary	100.00%
5.5% Senior Notes
Debt Instrument [Line Items]
Senior unsecured notes		$300,000,000	$0
5.5% Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate		5.50%	5.50%
5.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Senior unsecured notes	300,000,000
Stated interest rate	5.50%
Maximum percent of aggregate principal amount redeemable		35.00%
Events of default, period of payment default		30 days
Events of default, failure to comply with covenants, period after notice		180 days
Events of default, failure to comply with other agreements in the indenture, period after notice		30 days
Events of default, failure to pay final judgments, in excess of 20.0 million		20,000,000
Declaration of immediate payment, holding percent of principal amount required		25.00%
Debt defaults		0
7.5% Senior Notes
Debt Instrument [Line Items]
Senior unsecured notes		300,000,000	300,000,000
7.5% Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate		7.50%	7.50%
7.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Senior unsecured notes				300,000,000
Stated interest rate				7.50%
Maximum percent of aggregate principal amount redeemable		35.00%
Debt issuance costs				7,400,000
Events of default, period of payment default		30 days
Events of default, failure to comply with covenants, period after notice		180 days
Events of default, failure to comply with other agreements in the indenture, period after notice		30 days
Events of default, failure to pay final judgments, in excess of 20.0 million		20,000,000
Declaration of immediate payment, holding percent of principal amount required		25.00%
Debt defaults		0
Prior to August 15, 2017 \| 5.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		105.50%
August 15, 2017 - August 15, 2020 \| 5.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		104.13%
August 15, 2020 \| 5.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		100.00%
Prior to July 1, 2016 \| 7.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		107.50%
July 1, 2016 - June 30, 2019 \| 7.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		105.63%
July 1, 2019 \| 7.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Redemption price, expressed as percentage of principal amount		100.00%
Other noncurrent assets \| 5.5% Senior Notes \| Summit Holdings and Finance Corporation \| Senior Notes
Debt Instrument [Line Items]
Debt issuance costs	5,100,000

PARTNERS_CAPITAL_AND_MEMBERHIP2

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS - Partners' Capital and Schedule of Units (Details) (USD $)	12 Months Ended			1 Months Ended		0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Mar. 31, 2014	Oct. 03, 2012	Mar. 31, 2013
Class of Stock [Line Items]
Common units sold in public offering	5,408,337		14,380,000
Rollforwards of the number of partner units
Units, beginning balance	54,581,169	49,818,597	0
Units issued to the public in connection with public offering	5,408,337		14,380,000
Units issued to Summit Investments in connection with the IPO			35,436,020
Units issued under SMLP LTIP	47,508	5,892	2,577
Units, ending balance	60,037,014	54,581,169	49,818,597
Units withheld to meet the minimum statutory tax withholding requirement	14,300,000
Common
Class of Stock [Line Items]
Common units sold in public offering	5,300,000		14,380,000	4,347,826	10,350,000
Price per unit (dollars per unit)					$38.75
Rollforwards of the number of partner units
Units, beginning balance	29,079,866	24,412,427	0
Units issued to the public in connection with public offering	5,300,000		14,380,000	4,347,826	10,350,000
Units issued to Summit Investments in connection with the IPO			10,029,850
Units issued under SMLP LTIP	46,647	5,892	2,577
Units, ending balance	34,426,513	29,079,866	24,412,427
Subordinated
Class of Stock [Line Items]
Common units sold in public offering	0		0
Rollforwards of the number of partner units
Units, beginning balance	24,409,850	24,409,850	0
Units issued to the public in connection with public offering	0		0
Units issued to Summit Investments in connection with the IPO			24,409,850
Units issued under SMLP LTIP	0	0	0
Units, ending balance	24,409,850	24,409,850	24,409,850
General partner
Class of Stock [Line Items]
Common units sold in public offering	108,337		0
Rollforwards of the number of partner units
Units, beginning balance	1,091,453	996,320	0
Units issued to the public in connection with public offering	108,337		0
Units issued to Summit Investments in connection with the IPO			996,320
Units issued under SMLP LTIP	861	0	0
Units, ending balance	1,200,651	1,091,453	996,320
Partnership \| Common
Class of Stock [Line Items]
Common units sold in public offering					5,300,000
Rollforwards of the number of partner units
Units issued to the public in connection with public offering					5,300,000
Summit Investments \| Common
Class of Stock [Line Items]
Common units sold in public offering					5,050,000
Rollforwards of the number of partner units
Units issued to the public in connection with public offering					5,050,000
General partner
Class of Stock [Line Items]
General partner interest (as a percent)	2.00%	2.00%	2.00%		2.00%	2.00%	2.00%
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition						996,320
Bison Midstream
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		1,585,560
Bison Midstream \| Common
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		1,553,849
Bison Midstream \| Subordinated
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		0
Bison Midstream \| General partner
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		31,711
Mountaineer Midstream
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		3,171,120
Mountaineer Midstream \| Common
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		3,107,698
Mountaineer Midstream \| Subordinated
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		0
Mountaineer Midstream \| General partner
Rollforwards of the number of partner units
Units issued to affiliates in connection with acquisition		63,422

PARTNERS_CAPITAL_AND_MEMBERHIP3

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS - Allocation of Capital Distributions (Details) (USD $)	0 Months Ended	12 Months Ended			0 Months Ended					1 Months Ended
	Jun. 03, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 18, 2014	Jun. 04, 2013	Jun. 21, 2013	Jun. 04, 2013	Oct. 23, 2012	Jul. 31, 2012	31-May-12
Business Acquisition [Line Items]
Distributions to unitholders		($122,224,000)	($90,196,000)	($123,138,000)
Contributions to unitholders		4,235,000
Basis for determining number of units issued, period	5 days
Closing price (in dollars per unit)						$30.85		30.85
Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Percent of membership interest acquired					100.00%
Total cash consideration					305,000,000
Total consideration					307,941,000
Liabilities incurred for acquisition					2,900,000
Distributions to unitholders					-66,124,000
Bison Midstream
Business Acquisition [Line Items]
Total cash consideration						200,000,000
Total consideration						248,914,000
Contributions to unitholders						56,535,000
Basis for determining number of units issued, assumed equity issuance	50,000,000
Basis for determining number of units issued, period	5 days
Basis for determining number of units issued, weighted-average price (in dollars per unit)	31.53
Mountaineer Midstream
Business Acquisition [Line Items]
Total cash consideration							110,000,000
Cost of acquired entity, equity interests issued and issuable							100,000,000
Total consideration							210,000,000
Basis for determining number of units issued, assumed equity issuance	100,000,000
Basis for determining number of units issued, weighted-average price (in dollars per unit)	31.53
General partner
Business Acquisition [Line Items]
Distributions to unitholders		-4,770,000	-1,803,000
Contributions to unitholders		4,235,000		19,870,000
General partner \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Distributions to unitholders					-1,323,000
General partner \| Bison Midstream
Business Acquisition [Line Items]
Cost of acquired entity, equity interests issued and issuable						978,000
Units issued								31,711
Contributions to unitholders						1,131,000
General partner \| Mountaineer Midstream
Business Acquisition [Line Items]
Cost of acquired entity, equity interests issued and issuable							2,000,000
Units issued							63,422
Limited partners, Common
Business Acquisition [Line Items]
Distributions to unitholders		-67,658,000	-46,286,000	-64,178,000
Contributions to unitholders				211,938,000
Limited partners, Common \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Distributions to unitholders					-37,910,000
Limited partners, Common \| Bison Midstream
Business Acquisition [Line Items]
Cost of acquired entity, equity interests issued and issuable						47,936,000
Units issued								1,553,849
Contributions to unitholders						28,558,000
Limited partners, Common \| Mountaineer Midstream
Business Acquisition [Line Items]
Cost of acquired entity, equity interests issued and issuable							98,000,000
Units issued							3,107,698
Limited partners, Subordinated
Business Acquisition [Line Items]
Distributions to unitholders		-49,796,000	-42,107,000	-58,960,000
Contributions to unitholders				430,498,000
Limited partners, Subordinated \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Distributions to unitholders					-26,891,000
Limited partners, Subordinated \| Bison Midstream
Business Acquisition [Line Items]
Contributions to unitholders						26,846,000
Summit Investments \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Total consideration									206,694,000
Red Rock Gathering Company, LLC \| Summit Investments
Business Acquisition [Line Items]
Net investment					241,817,000
Bison Midstream \| Summit Investments
Business Acquisition [Line Items]
Net investment						305,449,000		305,449,000
Revolving credit facility
Business Acquisition [Line Items]
Amounts borrowed under revolving credit facility										50,000,000	163,000,000
Revolving credit facility \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Amounts borrowed under revolving credit facility					100,000,000
Revolving credit facility \| Mountaineer Midstream
Business Acquisition [Line Items]
Amounts borrowed under revolving credit facility							$110,000,000

PARTNERS_CAPITAL_AND_MEMBERHIP4

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS - Subordination and Cash Distribution Policy (Details) (USD $)	0 Months Ended	1 Months Ended		12 Months Ended
	Oct. 03, 2012	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Partners' Capital [Line Items]
Maximum period following end of quarter to distribute all available cash				45 days
General partner
Schedule of Partners' Capital [Line Items]
General partner interest (as a percent)	2.00%	2.00%	2.00%	2.00%	2.00%	2.00%
Condition one
Schedule of Partners' Capital [Line Items]
Minimum quarterly distribution (in dollars per share)				1.6
Term for condition to be met to end subordination period				3 years
Condition one \| General partner
Schedule of Partners' Capital [Line Items]
General partner interest (as a percent)				2.00%
Subordinated units
Schedule of Partners' Capital [Line Items]
Common unit per subordinated unit upon conversion at end of subordination period (in shares)				1

PARTNERS_CAPITAL_AND_MEMBERHIP5

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS - Minimum Quarterly Distribution (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Minimum quarterly distributions (in dollars per share)	$0.40
Minimum annual distributions (in dollars per share)	$1.60

PARTNERS_CAPITAL_AND_MEMBERHIP6

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS - Partnership Target Distributions (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Minimum quarterly distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.40
Thereafter
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.60
Minimum \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.40
Minimum \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.46
Minimum \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.50
Maximum \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.46
Maximum \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.50
Maximum \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Target quarterly distribution per unit target amount (in dollars per unit)	$0.60
Unitholders \| Minimum quarterly distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	98.00%
Unitholders \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	98.00%
Unitholders \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	85.00%
Unitholders \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	75.00%
Unitholders \| Thereafter
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	50.00%
General partner
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	2.00%
Quarterly distributions per unit, incentive threshold	$0.46
General partner \| Minimum quarterly distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	2.00%
General partner \| First target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	2.00%
General partner \| Second target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	15.00%
General partner \| Third target distribution
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	25.00%
General partner \| Thereafter
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions	50.00%
General partner \| Maximum
Schedule of Partnership Target Distributions [Line Items]
Percentage interest in distributions in excess of incentive threshold	50.00%

PARTNERS_CAPITAL_AND_MEMBERHIP7

PARTNERS' CAPITAL AND MEMBERHIP INTERESTS - Cash Distributions (Details) (USD $)	0 Months Ended								12 Months Ended			0 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Nov. 14, 2014	Aug. 14, 2014	15-May-14	Feb. 14, 2014	Nov. 14, 2013	Aug. 14, 2013	15-May-13	Feb. 14, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 22, 2015
Distribution Made to Limited Partner [Line Items]
Per-unit distribution (in dollars per unit)	$0.54	$0.52	$0.50	$0.48	$0.46	$0.44	$0.42	$0.41	$2.04	$1.73
Distributions to unitholders	$33,522	$31,953	$30,383	$26,366	$25,108	$23,740	$20,923	$20,425	$122,224	$90,196	$0
Subsequent Event
Distribution Made to Limited Partner [Line Items]
Per-unit distribution (in dollars per unit)												$0.56
General partner
Distribution Made to Limited Partner [Line Items]
General partner's IDRs	1,082	721	360	163	0	0	0	0
Limited partners, Common
Distribution Made to Limited Partner [Line Items]
Distributions to unitholders	18,589	17,900	17,211	13,958	13,377	12,647	10,253	10,009
Subordinated
Distribution Made to Limited Partner [Line Items]
Distributions to unitholders	13,181	12,693	12,205	11,717	11,229	10,618	10,252	10,008
General Partner Units
Distribution Made to Limited Partner [Line Items]
Distributions to unitholders	$670	$639	$607	$528	$502	$475	$418	$408

EARNINGS_PER_UNIT_Details

EARNINGS PER UNIT (Details) (USD $)	3 Months Ended									12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss) income										($21,164)	$53,304	$42,997
Less: net income attributable to the pre-IPO period										0	0	24,112
Less: net income attributable to Summit Investments										2,828	9,720	1,271
Net (loss) income attributable to SMLP	-37,686	6,113	4,036	3,545	16,345	6,691	8,068	12,480		-23,992	43,584	17,614
Less: net (loss) income attributable to general partner, including IDRs	689	1,204	801	431	490	134	161	250		3,125	1,035	352
Net (loss) income attributable to limited partners	-38,375	4,909	3,235	3,114	15,855	6,557	7,907	12,230		-27,117	42,549	17,262
(Loss) earnings per limited partner unit:
Antidilutive securities diluted excluded from the calculation of diluted loss per common unit									0	231,875	0
Common units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners										-16,324	23,227	8,632
Weighted-average units outstanding - basic										33,311,000	26,951,000	24,412,000
Less: effect of non-vested phantom units and non-vested restricted units (in units)										0	150,000	132,000
Weighted-average units outstanding b diluted										33,311,000	27,101,000	24,544,000
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	($0.65)	$0.08	$0.05	$0.08	$0.30	$0.12	$0.16	$0.25		($0.49)	$0.86	$0.35
Diluted (in dollars per share)	($0.65)	$0.08	$0.05	$0.08	$0.29	$0.12	$0.16	$0.25		($0.49)	$0.86	$0.35
Subordinated Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss) income attributable to limited partners										($10,793)	$19,322	$8,630
Weighted-average units outstanding - basic										24,410,000	24,410,000	24,410,000
Weighted-average units outstanding b diluted										24,410,000	24,410,000	24,410,000
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	($0.65)	$0.08	$0.05	$0.02	$0.30	$0.12	$0.16	$0.25		($0.44)	$0.79	$0.35
Diluted (in dollars per share)	($0.65)	$0.08	$0.05	$0.02	$0.30	$0.12	$0.16	$0.25		($0.44)	$0.79	$0.35

UNITBASED_COMPENSATION_Additio

UNIT-BASED COMPENSATION - Additional Disclosures (Details) (USD $)	7 Months Ended	12 Months Ended
	Apr. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2009
	holder	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2009
Long-term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized non-cash compensation expense		4,100,000
Remaining expected weighted-average vesting period of recognition of incremental non-cash compensation expense		2 years 3 months
Units forfeited (in shares)		0
Common Units \| Long-term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units reserved for issuance pursuant to LTIP		5,000,000
Units available for future issuance		4,600,000
Phantom Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units forfeited (in shares)		22,430	4,041
Remaining or outstanding interests		336,202
Phantom Units \| Long-term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		3 years
DFW Net Profit Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		4 years
Portion of membership interests authorized for issuance, percent					5.00%
Repurchase of DFW Net Profit Interest, number of holders	7
Payment to acquire vested net profit interests	12,200,000
Remaining or outstanding interests		0
Non-cash compensation expense			17,000	700,000
Restricted Units \| Long-term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted units exchanged for unvested net profit interests	7,393
SMP Net Profits Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		5 years
Portion of membership interests issued, percent				6.36%
Units forfeited (in shares)				0
Portion of membership interests authorized for issuance, percent					7.50%
Non-cash compensation expense			500,000	900,000
Red Rock Gathering Company, LLC \| DFW Net Profit Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-cash compensation expense			$500,000

UNITBASED_COMPENSATION_Phantom

UNIT-BASED COMPENSATION - Phantom and Restricted Unit Activity (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Phantom and Restricted units
Phantom and Restricted Unit Activity
Nonvested phantom and restricted units, beginning of period (in shares)	283,682			0
Unit vested (in shares)	-61,917
Nonvested phantom and restricted units, end of period (in shares)			131,558	0
Weighted-average grant date fair value
Nonvested, Weighted-average grant date fair value, beginning (in dollars per share)	$23.41			$0
Units vested, Weighted-average grant date fair value (in dollars per share)	$25.33
Nonvested, Weighted-average grant date fair value, ending (in dollars per share)			$20	$0
Phantom Units
Phantom and Restricted Unit Activity
Units granted (in shares)	136,867	155,330	125,000
Units forfeited (in shares)	-22,430	-4,041
Nonvested phantom and restricted units, end of period (in shares)	336,202
Weighted-average grant date fair value
Units granted, Weighted-average grant date fair value (in dollars per share)	$42.32	$26.33	$20
Units forfeited, Weighted-average grant date fair value (in dollars per share)	$25.56	$25.99
Nonvested, Weighted-average grant date fair value, ending (in dollars per share)	$30.61
Restricted Units
Phantom and Restricted Unit Activity
Units granted (in shares)		835	6,558
Weighted-average grant date fair value
Units granted, Weighted-average grant date fair value (in dollars per share)		$27.50	$20.23

UNITBASED_COMPENSATION_Vested_

UNIT-BASED COMPENSATION - Vested and Nonvested Interests (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
DFW Net Profit Interests
Percentage Interest
Nonvested at the beginning of the period (as a percent)	0.04%	1.75%
Repurchased (as a percent)	0.04%	0.00%
Vested (as a percent)	0.00%	1.64%
Forfeited (as a percent)	0.00%	0.07%
Nonvested at the end of the period (as a percent)	0.00%	0.04%
Vested, end of period (as a percent)	0.00%	4.29%
Weighted-average grant date fair value (per 1.0% of Net Profits Interests)
Nonvested at the beginning of the period (in dollars)	$1,650	$306
Repurchased (in dollars)	1,650	0
Vested (in dollars)	0	256
Forfeited (in dollars)	0	765
Nonvested at the end of the period (in dollars)	0	1,650
Vested, end of period (in dollars)	0	257
SMP Net Profits Interests
Percentage Interest
Nonvested at the beginning of the period (as a percent)		3.96%
Granted (as a percent)		0.50%
Vested (as a percent)		1.27%
Nonvested at the end of the period (as a percent)		3.19%
Vested, end of period (as a percent)		3.17%
Weighted-average grant date fair value (per 1.0% of Net Profits Interests)
Nonvested at the beginning of the period (in dollars)		1,003
Granted (in dollars)		1,780
Vested (in dollars)		965
Nonvested at the end of the period (in dollars)		1,140
Vested, end of period (in dollars)		$788

UNITBASED_COMPENSATION_Unitbas

UNIT-BASED COMPENSATION - Unit-based Compensation (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unit-based compensation	$4,696
Long-term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unit-based compensation	4,696	2,999	269
SMP Net Profits Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-cash compensation expense		500	900
DFW Net Profit Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-cash compensation expense		$17	$700

UNITBASED_COMPENSATION_Inputs_

UNIT-BASED COMPENSATION - Inputs Used in Option Pricing Method (Details) (SMP Net Profits Interests)	1 Months Ended
	Jan. 31, 2012
SMP Net Profits Interests
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Length of holding period restriction	2 years 11 months 5 days
Discount for lack of marketability (as a percent)	24.00%
Volatility (as a percent)	37.00%

CONCENTRATIONS_OF_RISK_Details

CONCENTRATIONS OF RISK (Details)	12 Months Ended
CONCENTRATIONS OF RISK (Details)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Counterparty A - Piceance Basin \| Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	22.00%	19.00%	27.00%
Counterparty A - Piceance Basin \| Accounts receivable \| Credit concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	29.00%	37.00%
Counterparty B - Williston Basin \| Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	10.00%		0.00%
Counterparty C - Williston Basin \| Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage			0.00%
Counterparty C - Williston Basin \| Accounts receivable \| Credit concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	14.00%
Counterparty D - Marcellus Shale \| Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage			0.00%
Counterparty D - Marcellus Shale \| Accounts receivable \| Credit concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage	10.00%
Counterparty E - Barnett Shale \| Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage		15.00%	19.00%
Counterparty E - Barnett Shale \| Accounts receivable \| Credit concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage		11.00%
Counterparty F - Barnett Shale \| Revenue \| Customer concentration
CONCENTRATIONS OF RISK
Concentration risk, percentage			14.00%

RELATEDPARTY_TRANSACTIONS_Deta

RELATED-PARTY TRANSACTIONS (Details) (USD $)	12 Months Ended			1 Months Ended
RELATED-PARTY TRANSACTIONS (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2012	31-May-12	Dec. 31, 2011	Oct. 27, 2011
RELATED-PARTY TRANSACTIONS
Interest paid in kind	$0	$0	$5,426,000
General partner \| Operation and maintenance expense allocation [Member]
RELATED-PARTY TRANSACTIONS
General and administrative expense allocation	17,223,000	13,808,000	2,913,000
General partner \| General and administrative expense allocation [Member]
RELATED-PARTY TRANSACTIONS
General and administrative expense allocation	18,843,000	17,362,000	3,661,000
Equipower
RELATED-PARTY TRANSACTIONS
Payments for service agreements with related party	234,000	199,000	204,000
IPS Engineering/EPC
RELATED-PARTY TRANSACTIONS
Payments for service agreements with related party	600,000	200,000
Sponsors
RELATED-PARTY TRANSACTIONS
Transaction expenses receivable						1,300,000
Reimbursements received for transaction expenses			300,000
Write-off of transaction expense receivable			1,000,000
Promissory Notes \| Sponsors
RELATED-PARTY TRANSACTIONS
Principal amount of debt							200,000,000
Stated interest rate							8.00%
Prepayments of notes				49,200,000	160,000,000
Interest paid in kind			6,300,000
Interest expense capitalized			900,000
Revolving credit facility
RELATED-PARTY TRANSACTIONS
Amounts borrowed under revolving credit facility				$50,000,000	$163,000,000

BENEFIT_PLAN_Details

BENEFIT PLAN (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]
Defined contribution benefit plan expense	$0.80	$0.60	$0.20

COMMITMENTS_AND_CONTINGENCIES_1

COMMITMENTS AND CONTINGENCIES (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Total rent expense related to operating leases	$1,677	$1,381	$732

ACQUISITIONS_Red_Rock_Gatherin

ACQUISITIONS - Red Rock Gathering (Details) (USD $)	12 Months Ended			0 Months Ended	3 Months Ended	0 Months Ended	1 Months Ended
ACQUISITIONS - Red Rock Gathering (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 18, 2014	Dec. 31, 2014	Oct. 23, 2012	Jul. 31, 2012	31-May-12
Business Acquisition [Line Items]
Purchase accounting adjustments	$1,185,000	$0	$0
Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Cost of acquired entity, cash paid				305,000,000
Borrowings incurred for acquisition				2,900,000
Cost of acquired entity, purchase price				307,941,000
Purchase accounting adjustments					1,200,000
Summit Investments \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Cost of acquired entity, purchase price						206,694,000
Cash						1,097,000
Accounts receivable						8,018,000
Other assets						317,000
Property, plant, and equipment						150,401,000
Rights-of-way						52,197,000
Other noncurrent assets						164,000
Total assets acquired						212,194,000
Trade accounts payable						2,558,000
Other current liabilities						2,942,000
Total liabilities assumed						5,500,000
Net identifiable assets acquired						206,694,000
Rights-of-way
Business Acquisition [Line Items]
Useful lives	24 years 2 months	24 years 4 months
Rights-of-way \| Summit Investments \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Useful lives	20 years
Revolving credit facility
Business Acquisition [Line Items]
Amounts borrowed under revolving credit facility							50,000,000	163,000,000
Revolving credit facility \| Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Amounts borrowed under revolving credit facility				$100,000,000

ACQUISITIONS_Lonestar_Assets_D

ACQUISITIONS - Lonestar Assets (Details) (Barnett Shale Play [Member], USD $)	0 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014
	contract
Barnett Shale Play [Member]
Property, Plant and Equipment [Line Items]
Payments to acquire gas gathering assets	$10.90
Number of long-term, fee-based contracts	2

ACQUISITIONS_Bison_Midstream_D

ACQUISITIONS - Bison Midstream (Details) (USD $)	12 Months Ended			0 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jun. 04, 2013	Feb. 15, 2013
Business Acquisition [Line Items]
Purchase price assigned to Bison Gas Gathering system	$303,168	$206,694
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill	115,888	45,478	61,689
SMP Holdings' equity in Bison Midstream
Business Acquisition [Line Items]
Purchase price assigned to Bison Gas Gathering system	303,168	206,694
Gas Gathering Contract \| Minimum
Business Acquisition [Line Items]
Useful lives			10 years
Gas Gathering Contract \| Maximum
Business Acquisition [Line Items]
Useful lives			25 years
Bison Midstream
Business Acquisition [Line Items]
Cost of acquired entity, purchase price				248,914
Cost of acquired entity, cash paid				200,000
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Purchase price					303,168
Current assets					5,705
Property, plant, and equipment					85,477
Intangible assets					164,502
Other noncurrent assets					2,187
Total assets acquired					257,871
Current liabilities					6,112
Other noncurrent liabilities					2,790
Total liabilities assumed					8,902
Net identifiable assets acquired					248,969
Goodwill					54,199
Bison Midstream \| Limited partners, Common
Business Acquisition [Line Items]
Cost of acquired entity, equity interests issued and issuable				47,900
Bison Midstream \| General Partner Units
Business Acquisition [Line Items]
Cost of acquired entity, equity interests issued and issuable				1,000
Bison Midstream \| SMP Holdings' equity in Bison Midstream
Business Acquisition [Line Items]
Cost of acquired entity, cash paid				200,000
Purchase price assigned to Bison Gas Gathering system				$303,200
Bison Midstream \| Gas Gathering Contract \| Minimum
Business Acquisition [Line Items]
Useful lives			5 years
Bison Midstream \| Gas Gathering Contract \| Maximum
Business Acquisition [Line Items]
Useful lives			15 years
Bison Midstream \| Gas Gathering Contract \| Weighted Average
Business Acquisition [Line Items]
Useful lives			12 years

ACQUISITIONS_Mountaineer_Midst

ACQUISITIONS - Mountaineer Midstream (Details) (USD $)	12 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jun. 21, 2013	Dec. 31, 2012
	Dec. 31, 2014	Dec. 31, 2013	compressor_station	Dec. 31, 2012
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Goodwill	61,689,000	115,888,000		$45,478,000
Contract intangibles
Business Acquisition [Line Items]
Useful lives	12 years 6 months	12 years 6 months
Mountaineer Midstream
Business Acquisition [Line Items]
Cost of acquired entity, purchase price			210,000,000
Number of compressor stations acquired			2
Cost of acquired entity, cash paid			110,000,000
Cost of acquired entity, equity interests issued and issuable			100,000,000
Mountaineer Midstream revenues since acquisition		9,600,000
Mountaineer Midstream net income since acquisition		2,300,000
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Purchase price			210,000,000
Property, plant, and equipment			163,661,000
Total assets acquired			193,789,000
Total liabilities assumed			0
Net identifiable assets acquired			193,789,000
Goodwill			16,211,000
Mountaineer Midstream \| Gas gathering agreement contract intangibles
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Intangible assets			24,019,000
Mountaineer Midstream \| Rights-of-way
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Intangible assets			6,109,000
Weighted Average \| Mountaineer Midstream
Business Acquisition [Line Items]
Useful lives			13 years
Scenario, Previously Reported \| Mountaineer Midstream
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Property, plant, and equipment			158,300,000
Goodwill			18,100,000
Scenario, Previously Reported \| Mountaineer Midstream \| Contract intangibles
Business Acquisition [Line Items]
Purchase price assigned, intangibles assets			27,100,000
Scenario, Previously Reported \| Mountaineer Midstream \| Rights-of-way
Business Acquisition [Line Items]
Purchase price assigned, intangibles assets			$6,500,000

ACQUISITIONS_Revenue_and_Net_I

ACQUISITIONS - Revenue and Net Income (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Revenues	$94,658	$79,030	$80,796	$76,202	$83,455	$76,019	$71,461	$61,984	$330,686	$292,920	$174,423
Net (loss) income									-21,164	53,304	42,997
Summit Midstream Partners, LP
Business Acquisition [Line Items]
Revenues									319,373	225,192	165,499
Net (loss) income									-23,992	43,584	41,726
Red Rock Gathering Company, LLC
Business Acquisition [Line Items]
Revenues									11,313	50,114	8,924
Net (loss) income									2,828	9,668	1,271
Bison Midstream, LLC
Business Acquisition [Line Items]
Revenues										17,614
Net (loss) income										$52

ACQUISITIONS_Pro_Forma_Details

ACQUISITIONS - Pro Forma (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended	1 Months Ended
ACQUISITIONS - Pro Forma (Details) (USD $)	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 03, 2012	Mar. 31, 2014	Mar. 31, 2013
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues	$94,658,000	$79,030,000	$80,796,000	$76,202,000	$83,455,000	$76,019,000	$71,461,000	$61,984,000	$330,686,000	$292,920,000	$174,423,000
Net (loss) income									-21,164,000	53,304,000	42,997,000
Pro forma total revenues									330,686,000	305,071,000	256,637,000
Pro forma net (loss) income									-20,938,000	47,371,000	38,639,000
Limited partners, Common
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Pro forma EPU, basic and diluted (in dollars per share)									($0.41)	$0.79	$0.28
Subordinated units
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Pro forma EPU, basic and diluted (in dollars per share)									($0.41)	$0.79	$0.28
Bison Midstream
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Incremental borrowings on revolving credit facility										310,000,000	310,000,000
Red Rock Gathering Company, LLC
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Incremental borrowings on revolving credit facility									100,000,000	100,000,000	100,000,000
Common unit issuance									5,300,000	5,300,000	5,300,000
Red Rock Gathering Company, LLC
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues									11,313,000	50,114,000	8,924,000
Net (loss) income									2,828,000	9,668,000	1,271,000
Red Rock Gathering Company, LLC \| Red Rock Gathering Company, LLC
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues									73,266,000	50,114,000	8,924,000
Net (loss) income									27,447,000	9,668,000	1,271,000
Bison Midstream and Mountaineer Midstream \| Bison Midstream and Mountaineer Midstream
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Revenues										60,323,000
Net (loss) income										-457,000
Acquisition-related Costs \| Red Rock Gathering Company, LLC
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Nonrecurring transaction cost									700,000		1,600,000
Acquisition-related Costs \| Bison Midstream and Mountaineer Midstream
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Nonrecurring transaction cost										2,500,000
General partner
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
General partner interest (as a percent)									2.00%	2.00%	2.00%	2.00%	2.00%	2.00%
Net (loss) income									$3,125,000	$1,035,000	$352,000

ACQUISITIONS_Grant_River_Gathe

ACQUISITIONS - Grant River Gathering (Details) (USD $)	12 Months Ended			3 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Business Acquisition [Line Items]
Purchase accounting adjustments	$1,185	$0	$0
Grand River Gathering, LLC
Business Acquisition [Line Items]
Purchase accounting adjustments				$1,200

UNAUDITED_QUARTERLY_FINANCIAL_2

UNAUDITED QUARTERLY FINANCIAL DATA (Details) (USD $)	3 Months Ended								9 Months Ended	12 Months Ended
UNAUDITED QUARTERLY FINANCIAL DATA (Details) (USD $)	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Limited Partners' Capital Account [Line Items]
Revenues	$94,658,000	$79,030,000	$80,796,000	$76,202,000	$83,455,000	$76,019,000	$71,461,000	$61,984,000		$330,686,000	$292,920,000	$174,423,000
Net (loss) income attributable to SMLP	-37,686,000	6,113,000	4,036,000	3,545,000	16,345,000	6,691,000	8,068,000	12,480,000		-23,992,000	43,584,000	17,614,000
Less: net income attributable to general partner, including IDRs	689,000	1,204,000	801,000	431,000	490,000	134,000	161,000	250,000		3,125,000	1,035,000	352,000
Net (loss) income attributable to limited partners	-38,375,000	4,909,000	3,235,000	3,114,000	15,855,000	6,557,000	7,907,000	12,230,000		-27,117,000	42,549,000	17,262,000
(Loss) earnings per limited partner unit:
Goodwill impairment	54,200,000								0	54,199,000	0	0
Long-lived asset impairment	5,500,000									5,505,000	0	0
Common units
Limited Partners' Capital Account [Line Items]
Net (loss) income attributable to limited partners										-16,324,000	23,227,000	8,632,000
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	($0.65)	$0.08	$0.05	$0.08	$0.30	$0.12	$0.16	$0.25		($0.49)	$0.86	$0.35
Diluted (in dollars per share)	($0.65)	$0.08	$0.05	$0.08	$0.29	$0.12	$0.16	$0.25		($0.49)	$0.86	$0.35
Subordinated units
Limited Partners' Capital Account [Line Items]
Net (loss) income attributable to limited partners										-10,793,000	19,322,000	8,630,000
(Loss) earnings per limited partner unit:
Basic (in dollars per share)	($0.65)	$0.08	$0.05	$0.02	$0.30	$0.12	$0.16	$0.25		($0.44)	$0.79	$0.35
Diluted (in dollars per share)	($0.65)	$0.08	$0.05	$0.02	$0.30	$0.12	$0.16	$0.25		($0.44)	$0.79	$0.35
Scenario, Previously Reported
Limited Partners' Capital Account [Line Items]
Revenues					69,298,000	63,096,000	59,285,000	43,595,000
Red Rock Gathering Company, LLC \| Adjustment
Limited Partners' Capital Account [Line Items]
Revenues					14,157,000	12,923,000	12,176,000	10,858,000
Bison Midstream \| Adjustment
Limited Partners' Capital Account [Line Items]
Revenues					$0	$0	$0	$7,531,000