Derivative Instruments - Cash Flow Hedges (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) number_of_interest_rate_swaps	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 12, 2018 number_of_interest_rate_swaps
Derivative instruments
Total interest expense line item on the income statements	$ 493.7	$ 501.5	$ 445.1
Reclassification to interest expense within next twelve months	5.5
WEC Energy Group
Derivative instruments
Total interest expense line item on the income statements	96.9	122.3	104.1
WEC Energy Group | WEC Energy Group 2007 Junior Notes due 2067
Derivative instruments
Long-term debt outstanding	$ 500.0
Interest rate swaps | WEC Energy Group
Derivative instruments
Number of interest rate swaps | number_of_interest_rate_swaps	2			2
Interest rate swap notional value	$ 250.0
Interest rate swap fixed interest rate	4.9765%
Derivative loss recognized in other comprehensive loss	$ (5.9)	(4.8)	(2.9)
Net derivative gain (loss) reclassified from accumulated other comprehensive loss to interest expense	$ (2.1)	$ 1.1	$ 1.6