| Standard label: | Coal Contract [Member] |
| Name: | us-gaap:CoalContractMember |
| Documentation: | Derivative instrument whose primary underlying risk is tied to coal. |
| Object type: | cm:Member |
| Data type: | [Member] (nonnum:domainItemType) |
| Period type: | For Period (duration) |
| Balance type: | (no balance type) |
| Abstract: | true |
| Workiva Wdesk Taxonomy Analyzer: | us-gaap:CoalContractMember |
| Label | Role | Language |
| Coal Contract [Member] | http://www.xbrl.org/2003/role/label | en-US |
| Reference | Role | Topic |