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

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Amount of unrealized gain (loss) related to the increase or decrease in fair value of interest rate derivatives designated as cash flow hedging instruments, which was recorded in accumulated other comprehensive income to the extent that the cash flow hedge was determined to be effective.

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