VARIABLE INTEREST ENTITIES - WEPCO ENVIRONMENTAL TRUST (Details) - USD ($) $ in Millions	1 Months Ended
	Nov. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Assets
Other current assets (restricted cash)		$ 4.6	$ 0.0
Regulatory assets		2,809.1	2,803.3
Other long-term assets (restricted cash)		0.6	0.0
Liabilities
Current portion of long-term debt		8.5	300.0
Long-term debt		2,867.1	$ 2,461.2
WEPCo Environmental Trust
Variable interest entities
Securitization of environmental control costs related to Pleasant Prairie power plant	$ 100.0
Assets
Other current assets (restricted cash)		4.6
Regulatory assets		102.9
Other long-term assets (restricted cash)		0.6
Liabilities
Current portion of long-term debt		8.5
Other current liabilities (accrued interest)		0.7
Long-term debt		$ 107.0