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PARTNERS' CAPITAL - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Jun. 29, 2018	May 31, 2018	Jul. 28, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 1,000
MGP | ARLP
Partners' capital
Ownership percentage by general partners				0.99%
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
Number of units retired				5,460,639
Repurchase price (in dollars per unit)				$ 17.12
Aggregate treasury units retired				$ 93,500
Contribution of units by affiliated entity (in units)					(467,018)
Exchange Transaction
Partners' capital
Common units issued in exchange				56,128,141
Exchange Transaction | MGP
Partners' capital
Common units issued in exchange			56,100,000
Exchange Transaction | SGP
Partners' capital
Common units issued in exchange			28,141
Exchange Transaction | MGP | ARLP
Partners' capital
Ownership percentage by general partners			0.99%
Exchange Transaction | SGP | ARLP
Partners' capital
Ownership percentage by general partners			0.01%
Exchange Transaction | General Partner's Capital (Deficit)
Partners' capital
Partners' capital, Exchange Transaction reclassification				$ 306,700
Exchange Transaction | Limited Partners' Capital
Partners' capital
Partners' capital, Exchange Transaction reclassification				$ (306,700)
Simplification Transactions
Partners' capital
Issuance of units to Owners of SGP in Simplification Transactions (in units)				1,322,388
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
Common units issued in exchange		29,188,997
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%
Simplification Transactions | Limited Partners' Capital
Partners' capital
Number of units retired				35

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The number of common units issued in an exchange transaction.

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