| Standard label: | Coal [Member] |
| Name: | us-gaap:PublicUtilitiesInventoryCoalMember |
| Documentation: | Combustible mineral substance consisting of carbonized vegetable matter used as fuel and in production of coal gas, water gas, and coal-tar compound. |
| 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:PublicUtilitiesInventoryCoalMember |
| Label | Role | Language |
| Coal [Member] | http://www.xbrl.org/2003/role/label | en-US |
| Reference | Role | Topic |
| FASB Accounting Standards Codification 210 10 S99 1 (SX 210.5-02(6)(a)(4)) | http://fasb.org/us-gaap/role/ref/legacyRef | 210 |
| FASB Accounting Standards Codification 606 10 55 91 (a) | http://www.xbrl.org/2009/role/commonPracticeRef | 606 |