REGULATORY ENVIRONMENT - 2020 and 2021 RATES (Details) - Public Service Commission of Wisconsin (PSCW) - USD ($) $ in Millions	1 Months Ended
	Dec. 31, 2019	Sep. 30, 2017
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%
Electric rates | 2020 rates
Public Utilities, General Disclosures [Line Items]
Approved rate increase	$ 15.8
Approved rate increase (as a percent)	1.60%
Electric rates | 2020 rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities	$ 11.0
Electric rates | 2021 rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities	39.0
Electric rates | 2020 and 2021 rates
Public Utilities, General Disclosures [Line Items]
Authorized revenue requirement for ReACT		$ 275.0
Cost of the ReACT project	$ 342.0
Electric rates | 2020 and 2021 rates | ReACT
Public Utilities, General Disclosures [Line Items]
Collection of ReACT regulatory asset in years	8 years
Electric rates | 2020 and 2021 rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization period	2 years
Electric rates | 2020 and 2021 rates | Earnings sharing mechanisms
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities	$ 21.0
Amortization period	2 years
Natural gas rates | 2020 rates
Public Utilities, General Disclosures [Line Items]
Approved rate increase	$ 4.3
Approved rate increase (as a percent)	1.40%
Natural gas rates | 2020 rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities	$ 5.0
Natural gas rates | 2021 rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization of regulatory liabilities	$ 5.0
Natural gas rates | 2020 and 2021 rates | Tax Cuts and Jobs Act of 2017
Public Utilities, General Disclosures [Line Items]
Amortization period	4 years