| Standard label: | Coal Supply Agreements [Member] |
| Name: | us-gaap:CoalSupplyAgreementsMember |
| Documentation: | Coal supply agreement under which an entity buys coal to meet the requirements of one of its facilities. |
| 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:CoalSupplyAgreementsMember |
| Label | Role | Language |
| Coal Supply Agreements [Member] | http://www.xbrl.org/2003/role/label | en-US |
| Reference | Role | Topic |
| FASB Accounting Standards Codification 805 20 55 31 (b) | http://fasb.org/us-gaap/role/ref/legacyRef | 805 |