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VARIABLE INTEREST ENTITIES - WEPCO ENVIRONMENTAL TRUST (Details) - USD ($) $ in Millions	1 Months Ended
	Nov. 30, 2020	Jun. 30, 2021	Dec. 31, 2020
Assets
Other current assets (restricted cash)		$ 16.8	$ 0.0
Regulatory assets (June 30, 2021 includes $106.0 related to WEPCo Environmental Trust)		3,409.9	3,524.1
Other long-term assets (restricted cash)		47.9	$ 47.8
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)		1.6
Regulatory assets (June 30, 2021 includes $106.0 related to WEPCo Environmental Trust)		106.0
Other long-term assets (restricted cash)		0.6
Liabilities
Current portion of long-term debt (June 30, 2021 includes $4.1 related to WEPCo Environmental Trust)		4.1
Other current liabilities (accrued interest)		0.3
Long-term debt		$ 111.1