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DERIVATIVE INSTRUMENTS - CASH FLOW HEDGES (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2021 USD ($) number_of_interest_rate_swaps | Mar. 31, 2020 USD ($)
Derivative instruments |  |
Interest expense | $ 119.5 | $ 129.4
WEC Energy Group | WEC Energy Group's junior subordinated notes due in 2067 |  |
Derivative instruments |  |
Long-term debt outstanding | $ 500.0 |
WEC Energy Group | Interest rate swaps |  |
Derivative instruments |  |
Number of interest rate swaps | number_of_interest_rate_swaps | 2 |
Interest rate swap notional value | $ 250.0 |
Interest rate swap fixed interest rate | 4.9765% |
Derivative loss recognized in other comprehensive loss | $ 0.0 | (4.7)
Net derivative loss reclassified from accumulated other comprehensive loss to interest expense | (1.4) | $ (0.1)
Amount to be reclassified from accumulated other comprehensive loss to interest expense | $ 3.8 |

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