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RATE MATTERS AND REGULATION - Proceedings (Details) $ in Millions | 1 Months Ended | 3 Months Ended
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May 31, 2019 USD ($) | Apr. 30, 2019 USD ($) | Jan. 31, 2019 USD ($) | Mar. 31, 2019
2018 Rate Case Proceedings | Westar Energy | Kansas Corporation Commission |  |  |  |
Regulatory Proceedings [Line Items] |  |  |  |
Refund percent on earnings over approved return on equity |  |  |  | 0.5
Approved return on equity, percentage |  |  |  | 9.30%
FERC 2019 TFR Proceeding | Westar Energy | Federal Energy Regulatory Commission |  |  |  |
Regulatory Proceedings [Line Items] |  |  |  |
Annual transmissions revenue estimated amount of increase (decrease) over prior year |  |  | $ (11.2) |
FERC 2019 TFR Proceeding | KCP&L | Federal Energy Regulatory Commission |  |  |  |
Regulatory Proceedings [Line Items] |  |  |  |
Annual transmissions revenue estimated amount of increase (decrease) over prior year |  |  | $ (2.8) |
Subsequent Event | Transmission Delivery Charge | Westar Energy | Kansas Corporation Commission |  |  |  |
Regulatory Proceedings [Line Items] |  |  |  |
Annual revenue decrease approved |  | $ 7.7 |  |
Scenario, Forecast | Subsequent Event | Transmission Delivery Charge | Westar Energy | Kansas Corporation Commission |  |  |  |
Regulatory Proceedings [Line Items] |  |  |  |
Annual transmissions revenue estimated amount of increase (decrease) over prior year | $ (8.3) |  |  |

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