PARTNERS' CAPITAL AND MEZZANINE CAPITAL - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended
	May 28, 2020	Mar. 31, 2021	Mar. 31, 2020	Apr. 15, 2021	Mar. 23, 2021	Mar. 10, 2021	Dec. 31, 2020	Dec. 31, 2017
Partners Capital [Line Items]
Preferred units issued		162,109					162,109
Common limited partner capital (in shares), outstanding		6,206,021					6,110,092
Common Units Issued					2,400,000	2,160,000
Number of Common Units						$ 27
Exchange Offer Expired Date		Apr. 13, 2021
Subsidiary Series A preferred unitholders, outstanding		162,109					162,109
Number of Common Units Exchanged		$ 30
Common Units Net of Units Withheld for Withholding Taxes		538,715
Maximum period following end of quarter to distribute all available cash		45 days
Common Units
Partners Capital [Line Items]
Number of common units purchased.	537,307
Exercise price of Warrants	$ 15.345		$ 1.025
Valuation of warrants		$ 3.4
Warrant or right, reason for issuance, description		Upon exercise of the ECP Warrants, each of ECP NewCo and ECP Holdings may receive, at its election: (i) a number of SMLP common units equal to the number of SMLP common units for which the ECP Warrants are being exercised, if exercising the ECP Warrants by cash payment of the exercise price; (ii) a number of SMLP common units equal to the product of the number of common units being exercised multiplied by (a) the difference between the average of the daily volume-weighted average price (“VWAP”) of the SMLP common units on the NYSE on each of the three trading days prior to the delivery of the notice of exercise (the “VWAP Average”) and the exercise price (the “VWAP Difference”), divided by (b) the VWAP Average; and/or (iii) an amount in cash, to the extent that the payment of such cash would not result in any violation of any financial covenant under the Revolving Credit Facility, and the Partnership’s leverage ratio would be at least 0.5x less than the maximum applicable ratio set forth in the Revolving Credit Facility, equal to the product of (a) the number of SMLP common units exercised and (b) the VWAP Difference, subject to certain adjustments under the ECP Warrants.
Common Units | ECP Holdings Warrant
Partners Capital [Line Items]
Number of common units purchased.	129,360		1,940,391
Common Units | Energy Capital Partners
Partners Capital [Line Items]
Warrants exercisable date	May 28, 2023
Maximum | Common Units
Partners Capital [Line Items]
Common Units Issued	666,667		10,000,000
Exercise price of Warrants	$ 30.00		$ 2.00
Series A Preferred Units
Partners Capital [Line Items]
Preferred units issued		18,662						300,000
Interest in partnership per unit								$ 1,000
Common limited partner capital (in shares), outstanding		162,109
Redemption amount		$ 108.5
Series A Preferred Units | PIK [Member]
Partners Capital [Line Items]
Preferred units issued		1,494
Subsidiary Series A preferred unitholders, outstanding		86,802
Series A Preferred Units | Subsequent Event
Partners Capital [Line Items]
Subsidiary Series A preferred unitholders, outstanding				143,447