REGULATORY ENVIRONMENT (Details) $ in Millions	1 Months Ended			6 Months Ended
	May 31, 2018	Dec. 31, 2017 USD ($)	Sep. 30, 2017	Jun. 30, 2018 USD ($) customer	Jun. 30, 2017
Regulatory environment
Corporate federal tax rate (as a percent)				21.00%	35.00%
Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Approved return on equity (as a percent)			10.20%
Percentage of first 50 basis points of additional utility earnings shared with customers			50.00%
Return on equity in excess of authorized amount (as a percent)			0.50%
Michigan Public Service Commission (MPSC)
Regulatory environment
Number of retail electric customers served in Michigan | customer				1
Tax Cuts And Jobs Act Of 2017
Regulatory environment
Change in deferred taxes from tax legislation		$ 1,065.0
Electric rates | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Percentage of current tax benefit from Tax Cuts and Jobs Act of 2017 to be used to reduce certain regulatory assets	80.00%
Percent of current tax benefit from Tax Cuts and Jobs Act of 2017 to be returned to customers via bill credits	20.00%
Decrease in Regulatory Assets Related to PSCW Tax Legislation Ruling				$ 33.8
Natural gas rates | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Percent of current tax benefit from Tax Cuts and Jobs Act of 2017 to be returned to customers via bill credits	100.00%