| Standard label: | Increase (Decrease) in Coal Inventories |
| Name: | us-gaap:IncreaseDecreaseInCoalInventories |
| Documentation: | The increase (decrease) during the reporting period in the value of coal held in inventory. |
| Object type: | cm:Concept |
| Data type: | Monetary (xbrli:monetaryItemType) |
| Period type: | For Period (duration) |
| Balance type: | Credit |
| Abstract: | false |
| Workiva Wdesk Taxonomy Analyzer: | us-gaap:IncreaseDecreaseInCoalInventories |
| Label | Role | Language |
| Increase (Decrease) in Coal Inventories | http://www.xbrl.org/2003/role/label | en-US |
| Reference | Role | Topic |
| FASB Accounting Standards Codification 230 10 45 28 (a) | http://fasb.org/us-gaap/role/ref/legacyRef | 230 |
| Part Of Roll Up: |
| us-gaap:IncreaseDecreaseInFossilFuelInventories |