| Standard label: | Asset Retirement Obligation and Environmental Cost [Policy Text Block] |
| Name: | us-gaap:AssetRetirementObligationsAndEnvironmentalCostPolicyTextBlock |
| Documentation: | Disclosure of accounting policy for legal obligation associated with retirement of long-lived asset that results from acquisition, construction, or development or from normal operation of long-lived asset, and environmental remediation liability from improper or other-than-normal operation of long-lived asset. Includes, but is not limited to, timing of recognition and whether obligation is measured on discounted basis. |
| Object type: | cm:Level2TextBlock |
| Data type: | Text Block (HTML) (nonnum:textBlockItemType) |
| Period type: | For Period (duration) |
| Balance type: | (no balance type) |
| Abstract: | false |
| Workiva Wdesk Taxonomy Analyzer: | us-gaap:AssetRetirementObligationsAndEnvironmentalCostPolicyTextBlock |
| Label | Role | Language |
| Asset Retirement Obligation and Environmental Cost [Policy Text Block] | http://www.xbrl.org/2003/role/label | en-US |
| Reference | Role | Topic |
| FASB Accounting Standards Codification 235 10 50 3 | http://fasb.org/us-gaap/role/ref/legacyRef | 235 |
| FASB Accounting Standards Codification 410 20 50 1 | http://fasb.org/us-gaap/role/ref/legacyRef | 410 |
| FASB Accounting Standards Codification 410 30 50 8 | http://www.xbrl.org/2003/role/recommendedDisclosureRef | 410 |
| FASB Accounting Standards Codification 410 30 55 14 | http://www.xbrl.org/2003/role/exampleRef | 410 |