Derivative Instruments - Cash Flow Hedges (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) number_of_interest_rate_swaps	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative instruments
Total interest expense line item on the income statements	$ 501.5	$ 445.1	$ 415.7
Reclassification to interest expense within next 12 months	1.0
WEC Energy Group
Derivative instruments
Total interest expense line item on the income statements	122.3	104.1	82.0
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 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 other comprehensive loss	$ (4.8)	(2.9)	0.0
Net derivative gains reclassified from accumulated other comprehensive loss to interest expense	$ 1.1	$ 1.6	$ 2.2