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DERIVATIVE INSTRUMENTS - CASH FLOW HEDGES (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) number_of_interest_rate_swaps	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) number_of_interest_rate_swaps	Sep. 30, 2019 USD ($)
Derivative instruments
Interest expense	$ 122.0	$ 125.8	$ 375.8	$ 374.3
WEC Energy Group | WEC Energy Group's junior subordinated notes due in 2067
Derivative instruments
Long-term debt outstanding	$ 500.0		$ 500.0
WEC Energy Group | Interest rate swaps
Derivative instruments
Number of interest rate swaps | number_of_interest_rate_swaps	2		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	$ 0.0	(0.5)	$ (5.8)	(5.3)
Net derivative gains (losses) reclassified from accumulated other comprehensive loss to interest expense	(1.3)	$ 0.2	(2.0)	$ 1.0
Amount to be reclassified from accumulated other comprehensive loss to interest expense	$ 5.1		$ 5.1

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