REGULATORY ENVIRONMENT (Details) $ in Millions	1 Months Ended			9 Months Ended
	May 31, 2018 USD ($) MW solar_projects	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018	Sep. 30, 2017
Regulatory environment
Corporate federal tax rate (as a percent)				21.00%	35.00%
Public Service Commission of Wisconsin (PSCW) | Badger Hollow and Two Creeks Solar Farms
Regulatory environment
Number of solar projects in Wisconsin that WPS filed with the PSCW to acquire an interest in | solar_projects	2
WPS total ownership in two solar projects (in megawatts) | MW	200
WPS share of cost for certain solar projects	$ 260.0
Public Service Commission of Wisconsin (PSCW) | Badger Hollow Solar Farm
Regulatory environment
WPS individual ownership in a solar project (in megawatts) | MW	100
Public Service Commission of Wisconsin (PSCW) | Two Creeks Solar Project
Regulatory environment
WPS individual ownership in a solar project (in megawatts) | MW	100
Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Approved return on equity (as a percent)			10.00%
Authorized revenue requirement for the ReACT project			$ 275.0
AFUDC			51.0
Estimated cost of the ReACT project, excluding AFUDC			$ 342.0
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%
Tax Cuts And Jobs Act Of 2017
Regulatory environment
Change in deferred taxes from tax legislation		$ 444.7
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	40.00%
Percent of current tax benefit from Tax Cuts and Jobs Act of 2017 to be returned to customers via bill credits	60.00%
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%