REGULATORY ENVIRONMENT (Details) $ in Millions	1 Months Ended
	Oct. 31, 2017 USD ($) MW	Sep. 30, 2017 USD ($) utility Assurance	Jan. 31, 2017	Oct. 31, 2016 USD ($)	Apr. 30, 2015 USD ($)	Jun. 30, 2017 USD ($)
WE | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Approved return on equity (as a percent)		10.20%
WG | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Approved return on equity (as a percent)		10.30%
WPS | 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
WE, WG, and WPS | Public Service Commission of Wisconsin (PSCW) | Natural gas storage facilities in Michigan
Regulatory environment
Percentage of natural gas storage needs provided by the facilities			33.33%
WE, WG, and WPS | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Number of utilities with earnings sharing mechanism | utility		3
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%
PGL | Illinois Commerce Commission (ICC)
Regulatory environment
Amount of assurance that PGL's QIP rider costs will be recoverable | Assurance		0
MERC | Minnesota Public Utilities Commission (MPUC) | 2018 rates | Natural gas rates | Subsequent event
Regulatory environment
Requested rate increase	$ 12.6
Requested rate increase (as a percent)	5.05%
Requested return on equity (as a percent)	10.30%
Requested common equity component average (as a percent)	50.90%
MERC | Minnesota Public Utilities Commission (MPUC) | 2016 rates | Natural gas rates
Regulatory environment
Approved return on equity (as a percent)				9.11%
Approved rate increase				$ 6.8
Approved rate increase (as a percent)				3.00%
Approved common equity component average (as a percent)				50.32%
Number of years decoupling mechanism authorized for use				3 years
Interim rate refund						$ 4.1
UMERC | Subsequent event
Regulatory environment
Capacity of natural gas-fired generation facility (in megawatts) | MW	180
UMERC | Michigan Public Service Commission (MPSC) | Subsequent event
Regulatory environment
Term of electric power purchase agreement (in years)	20 years
Capacity of natural gas-fired generation facility (in megawatts) | MW	180
Estimated cost of constructing a power plant	$ 265.7
Estimated cost of constructing a power plant, including AFUDC	$ 277.0
Portion of the power plant costs recoverable from Tilden Mining Company (as a percent)	50.00%
Portion of the power plant costs recoverable from utility customers (as a percent)	50.00%
UMERC | Michigan Public Service Commission (MPSC) | 2015 rates | Electric rates
Regulatory environment
Approved return on equity (as a percent)					10.20%
Approved rate increase					$ 4.0
Approved common equity component average (as a percent)					50.48%
Period of rate implementation (in years)					3 years