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DERIVATIVE INSTRUMENTS - CASH FLOW HEDGES (Details) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) number_of_interest_rate_swaps | Jun. 30, 2018 USD ($)
Derivative [Line Items] |  |  |  |
Interest expense | $ 124.1 | $ 108.5 | $ 248.5 | $ 215.2
WEC Energy Group | WEC 2007 Junior Notes due 2067 |  |  |  |
Derivative [Line Items] |  |  |  |
Long-term debt outstanding | 500.0 |  | $ 500.0 |
WEC Energy Group | Interest rate swaps |  |  |  |
Derivative [Line Items] |  |  |  |
Number of interest rate swaps executed | number_of_interest_rate_swaps |  |  | 2 |
Interest rate swap notional value | $ 250.0 |  | $ 250.0 |
Interest rate swap fixed interest rate | 4.9765% |  | 4.9765% |
Derivative losses recognized in other comprehensive loss | $ (3.2) | 0.0 | $ (4.8) | 0.0
Net derivative gains reclassified from accumulated other comprehensive loss to interest expense | 0.4 | $ 0.5 | 0.8 | $ 1.1
Amount to be reclassified from accumulated other comprehensive loss to interest expense | $ 0.3 |  | $ 0.3 |

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