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COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Oct. 31, 2014 compliance_option | Mar. 31, 2018 USD ($) degreecelsius MW | Dec. 31, 2017 USD ($) T | Dec. 31, 2015 | Sep. 30, 2016 Case
Climate Change | Electric |  |  |  |  |
Air quality |  |  |  |  |
Number of legal cases heard by a court | Case |  |  |  |  | 1
Percentage of nationwide greenhouse gas emissions reduction |  |  |  | 32.00% |
Interim goal for greenhouse gas emissions reduction (fraction) |  |  |  | 0.667 |
Company goal for percentage of carbon dioxide emissions reduction |  | 40.00% |  |  |
Coal Generation Capacity Expected To Be Retired By 2020 | MW |  | 1,800 |  |  |
Carbon Dioxide Emissions | T |  |  | 29.2 |  |
Climate Change | Electric | Maximum |  |  |  |  |
Air quality |  |  |  |  |
Global temperature increases limit | degreecelsius |  | 2 |  |  |
Climate Change | Electric | Wisconsin |  |  |  |  |
Air quality |  |  |  |  |
Percentage of greenhouse gas emissions reduction by state |  |  |  | 41.00% |
Climate Change | Electric | Michigan |  |  |  |  |
Air quality |  |  |  |  |
Percentage of greenhouse gas emissions reduction by state |  |  |  | 39.00% |
Climate Change | Natural gas |  |  |  |  |
Air quality |  |  |  |  |
Carbon Dioxide Emissions | T |  |  | 26.6 |  |
Clean Water Act Cooling Water Intake Structure Rule | Electric |  |  |  |  |
Water quality |  |  |  |  |
Number of compliance options available to meet standard | compliance_option | 7 |  |  |  |
Renewal period for facility permits |  | 5 years |  |  |
Steam Electric Effluent Limitation Guidelines | Electric |  |  |  |  |
Water quality |  |  |  |  |
Renewal period for facility permits |  | 5 years |  |  |
Steam Electric Effluent Limitation Guidelines | Electric | Minimum |  |  |  |  |
Water quality |  |  |  |  |
Expected cost to achieve required emissions reductions |  | $ 70.0 |  |  |
Manufactured Gas Plant Remediation | Natural gas |  |  |  |  |
Manufactured gas plant remediation |  |  |  |  |
Regulatory assets recorded for remediation of manufactured gas plant sites |  | 668.2 | $ 676.6 |  |
Reserves recorded for remediation of manufactured gas plant sites |  | $ 617.2 | $ 617.2 |  |

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