LONG-TERM DEBT - Credit Facility (Details) $ in Thousands		3 Months Ended
Mar. 09, 2020 USD ($)	Mar. 31, 2021 USD ($)
Credit Agreement
Long-Term Debt
Maximum borrowing capacity	$ 537,750
ARLP debt arrangements requirements, fixed charge coverage ratio	1.0
ARLP debt arrangements requirements, debt to cash flow ratio	2.5
ARLP debt arrangements requirements, cash flow to interest expense ratio	3.0
Actual debt to cash flow ratio for trailing twelve months		1.43
Actual cash flow to interest expense ratio for trailing twelve months		8.67
ARLP debt arrangements requirements, period over which the ratios are required to be maintained	12 months
Credit Agreement \| Eurodollar Rate
Long-Term Debt
Effective interest rate (as a percent)		2.97%
Credit Agreement \| Extended Commitment to May 23, 2021
Long-Term Debt
Maximum borrowing capacity	$ 459,500
Credit Agreement \| Credit Agreement, first lien
Long-Term Debt
ARLP debt arrangements requirements, debt to cash flow ratio	1.5
Actual debt to cash flow ratio for trailing twelve months		0.39
Revolving credit facility
Long-Term Debt
Line of credit facility, available for borrowing		$ 461,000
Annual commitment fee percentage, undrawn portion		0.35%
Letters of credit subfacility
Long-Term Debt
Maximum borrowing capacity	$ 125,000
Letters of credit outstanding		$ 21,800
Swingline subfacility
Long-Term Debt
Maximum borrowing capacity	$ 15,000

Represents the ratio of cash flow to interest expense.

No definition available.

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- Definition

Represents the period over which the ratios required under the terms of the debt agreement are calculated.

+ References

No definition available.

+ Details

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- Definition

Represents the minimum cash flow to interest expense ratio required under the terms of the debt agreement.

+ References

No definition available.

+ Details

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- Definition

Represents the maximum debt to cash flow ratio required under the terms of the debt agreement.

+ References

No definition available.

+ Details

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Namespace Prefix:	arlp_
Data Type:	xbrli:pureItemType
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- Definition

Represents the fixed charge coverage ratio, after distributions, required under the terms of the debt agreement.

+ References

No definition available.

+ Details

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Data Type:	xbrli:pureItemType
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- Definition

Represents the ratio of debt to cash flow.

+ References

No definition available.

+ Details

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Namespace Prefix:	arlp_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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Period Type:	duration

- Details

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- Details

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