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PARTNERS' CAPITAL - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 29, 2018	May 31, 2018	Dec. 31, 2020	Dec. 31, 2018
Partners' capital
Cash contribution by affiliated entity				$ 2,142
Affiliated entity controlled by Mr. Craft
Partners' capital
Contribution of units by affiliated entity (in units)	467,018
Cash contribution by affiliated entity				$ 2,100
MGP | Intermediate Partnership
Partners' capital
Ownership percentage by general partners			1.0001%
MGP | Alliance Coal
Partners' capital
Ownership percentage by general partners			0.001%
Limited Partners' Capital
Partners' capital
Issuance of units to Owners of SGP in Simplification Transactions (in units)				1,322,388
Repurchase and retire authorization		$ 100,000
Treasury units retired			$ 93,500
Number of units retired			5,460,639
Repurchase price (in dollars per unit)			$ 17.12
Contribution of units by affiliated entity (in units)				(467,018)
Simplification Transactions | Intermediate Partnership
Partners' capital
Ownership percentage by general partners		1.0001%
Simplification Transactions | Alliance Coal
Partners' capital
Ownership percentage by general partners		0.001%
Simplification Transactions | SGP
Partners' capital
Issuance of units to Owners of SGP in Simplification Transactions (in units)		1,322,388
Simplification Transactions | MGP | Intermediate Partnership
Partners' capital
Ownership percentage by general partners		1.0001%
Simplification Transactions | MGP | Alliance Coal
Partners' capital
Ownership percentage by general partners		0.001%
Simplification Transactions | SGP | Intermediate Partnership
Partners' capital
Ownership percentage by general partners		1.0001%
Simplification Transactions | SGP | Alliance Coal
Partners' capital
Ownership percentage by general partners		0.001%

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