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DERIVATIVE INSTRUMENTS - CASH FLOW HEDGES (Details) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 USD ($) number_of_interest_rate_swaps | Mar. 31, 2018 USD ($)
Derivative [Line Items] |  |
Interest expense | $ 124.4 | $ 106.7
Interest rate swaps |  |
Derivative [Line Items] |  |
Amount to be reclassified from accumulated OCI to interest expense | 1.5 |
WEC Energy Group | WEC 2007 Junior Notes due 2067 |  |
Derivative [Line Items] |  |
Long-term debt outstanding | $ 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 |
Interest rate swap fixed interest rate | 4.9765% |
Derivative losses recognized in OCI | $ (1.6) | 0.0
Net derivative gains reclassified from accumulated OCI to interest expense | $ 0.4 | $ 0.6

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