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Note 15 - Merom Acquisition (Details Textual) - USD ($) $ in Millions | Feb. 14, 2022 | May 31, 2023
--- | --- | ---
Capacity Payment Reductions |  | $ 7.5
Merom Acquisition [Member] |  |
Capacity Payment Reductions | $ 15.0 |
Purchase Obligation, Total | 7.2 |
Business Acquisition, Purchase of Coal Inventory | 17.0 |
Business Acquisition, Purchase of Coal Inventory, Initial Payment | 5.4 |
Business Acquisition, Transaction Costs | $ 2.9 |

X
- Definition Represents purchase of coal inventory in a business acquisition transaction. + References No definition available. + Details
Name: | hnrg_BusinessAcquisitionPurchaseOfCoalInventory
Namespace Prefix: | hnrg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents initial payment for purchase of coal inventory in a business acquisition transaction. + References No definition available. + Details
Name: | hnrg_BusinessAcquisitionPurchaseOfCoalInventoryInitialPayment
Namespace Prefix: | hnrg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents the value in reduction to capacity payments. + References No definition available. + Details
Name: | hnrg_CapacityPaymentReductions
Namespace Prefix: | hnrg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of direct costs of the business combination including legal, accounting, and other costs incurred to consummate the business acquisition. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionCostOfAcquiredEntityTransactionCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier. + References No definition available. + Details
Name: | us-gaap_PurchaseObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=hnrg_MeromAcquisitionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |