REGULATORY ENVIRONMENT - 2020 AND 2021 RATES (Details) - USD ($) $ in Millions	1 Months Ended
	May 31, 2021	Nov. 30, 2020	Dec. 31, 2019
WEPCo Environmental Trust Finance I, LLC | WEPCo Environmental Trust Bonds 1.578%, due 2035
Public Utilities, General Disclosures [Line Items]
Proceeds from issuance of debt	$ 118.8
Interest rate on long-term debt	1.578%
Public Service Commission of Wisconsin (PSCW) | WEPCo Environmental Trust Finance I, LLC | WEPCo Environmental Trust Bonds 1.578%, due 2035
Public Utilities, General Disclosures [Line Items]
Proceeds from issuance of debt	$ 118.8
Interest rate on long-term debt	1.578%
Public Service Commission of Wisconsin (PSCW) | 2020 rates | Electric rates
Public Utilities, General Disclosures [Line Items]
Approved rate increase			$ 15.3
Approved rate increase (as a percent)			0.50%
Public Service Commission of Wisconsin (PSCW) | 2020 rates | Electric rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities			$ 65.0
Public Service Commission of Wisconsin (PSCW) | 2020 rates | Natural gas rates
Public Utilities, General Disclosures [Line Items]
Approved rate increase			$ 10.4
Approved rate increase (as a percent)			2.80%
Public Service Commission of Wisconsin (PSCW) | 2020 rates | Natural gas rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities			$ (5.0)
Public Service Commission of Wisconsin (PSCW) | 2020 rates | Steam rates
Public Utilities, General Disclosures [Line Items]
Approved rate increase			$ 1.9
Approved rate increase (as a percent)			8.60%
Public Service Commission of Wisconsin (PSCW) | 2021 rates | Electric rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities			$ 65.0
Public Service Commission of Wisconsin (PSCW) | 2021 rates | Natural gas rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities			$ (5.0)
Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates
Public Utilities, General Disclosures [Line Items]
Approved return on equity (as a percent)			10.00%
Approved common equity component average (as a percent)			52.50%
Percentage of first 25 basis points of additional earnings retained by the utility			100.00%
Return on equity in excess of authorized amount (as a percent)			0.25%
Percentage of additional earnings between 25 and 75 basis points refunded to customers			50.00%
Return on equity in excess of first 25 basis points above authorized amount (as a percent)			0.50%
Percentage of earnings in excess of 75 basis points refunded to customers			100.00%
Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates | Electric rates
Public Utilities, General Disclosures [Line Items]
Pleasant Prairie power plant's book value to be securitized		$ 100.0
Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates | Electric rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization period			2 years
Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates | Natural gas rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization period			4 years