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COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) $ in Millions | 1 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Oct. 31, 2014 | Jun. 30, 2017 USD ($) | Dec. 31, 2015 | Dec. 31, 2016 USD ($) | Sep. 30, 2016
Climate Change | Electric |  |  |  |  |
Air quality |  |  |  |  |
Number of legal cases heard by a court |  |  |  |  | 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% |  |  |
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% |  |
Clean Water Act Cooling Water Intake Structure Rule | Electric |  |  |  |  |
Water quality |  |  |  |  |
Number of compliance options available to meet standard | 7 |  |  |  |
Steam Electric Effluent Limitation Guidelines | Electric |  |  |  |  |
Water quality |  |  |  |  |
Renewal period for facility permits |  | 5 years |  |  |
Steam Electric Effluent Limitation Guidelines | Minimum | Electric |  |  |  |  |
Water quality |  |  |  |  |
Expected cost to achieve required emissions reductions |  | $ 80.0 |  |  |
Steam Electric Effluent Limitation Guidelines | Maximum | Electric |  |  |  |  |
Water quality |  |  |  |  |
Expected cost to achieve required emissions reductions |  | 110.0 |  |  |
Manufactured Gas Plant Remediation | Natural gas |  |  |  |  |
Manufactured gas plant remediation |  |  |  |  |
Regulatory assets recorded for remediation of manufactured gas plant sites |  | 683.8 |  | $ 702.7 |
Reserves recorded for remediation of manufactured gas plant sites |  | $ 622.4 |  | $ 633.4 |

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