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COMMITMENTS AND CONTINGENCIES - Purchase Commitments (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Mar. 09, 2018
--- | --- | ---
Contractual Commitments |  |
Commitments to purchase coal |  | $ 2.0
Commitments for external coal purchases |  |
Contractual Commitments |  |
Commitments to purchase coal | $ 0.0 |
Commitments related to planned capital projects |  |
Contractual Commitments |  |
Contractual amount | $ 21.0 |

X
- Definition Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments. + References No definition available. + Details
Name: | us-gaap_ContractualObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of purchase arrangement to be paid in next fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
Name: | us-gaap_PurchaseObligationDueInNextTwelveMonths
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis=arlp_ExternalCoalPurchaseCommitmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=us-gaap_CapitalAdditionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |