REGULATORY ENVIRONMENT (Details) $ in Millions	1 Months Ended									3 Months Ended
	Apr. 30, 2018 utility	Mar. 31, 2018 USD ($) customer Assurance	Feb. 28, 2018 USD ($) day Filings	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($) MW	Sep. 30, 2017 USD ($) utility	Jan. 31, 2017	Aug. 31, 2016 MW	Mar. 31, 2018 customer change Assurance	Mar. 31, 2017	Apr. 02, 2018 wind_turbines MW
Regulatory environment
Corporate federal tax rate										21.00%	35.00%
Forward Wind Energy Center | Subsequent event
Regulatory environment
Number of wind turbines at Forward Wind Energy Center | wind_turbines												86
Capacity of Foward Wind Energy Center | MW												129
WE | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Approved return on equity (as a percent)							10.20%
Income Statement impact of flow through of repair related deferred tax liabilities | change										0
WE | Michigan Public Service Commission (MPSC)
Regulatory environment
Number of retail electric customers served in Michigan | customer		1								1
WG | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates
Regulatory environment
Approved return on equity (as a percent)							10.30%
WPS | Forward Wind Energy Center | Subsequent event
Regulatory environment
Number of utilities along with WPS that entered in an agreement to purchase Forward Wind Energy Center | utility	2
WPS | Public Service Commission of Wisconsin (PSCW) | Forward Wind Energy Center | Subsequent event
Regulatory environment
Number of utilities along with WPS that entered in an agreement to purchase Forward Wind Energy Center | utility	2
Number of wind turbines at Forward Wind Energy Center | wind_turbines												86
Capacity of Foward Wind Energy Center | MW												129
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%
WE, WG, and WPS | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates | Natural gas rates | Subsequent event
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%
WE and WPS | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates | Electric rates | Subsequent event
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%
PGL | Illinois Commerce Commission (ICC)
Regulatory environment
Amount of rate base reduction as a result of a settlement			$ 5.4
Amount to be refunded to ratepayers as a result of a settlement			$ 4.7
Amount of assurance that PGL's QIP rider costs will be recoverable | Assurance		0								0
MGU and UMERC | Michigan Public Service Commission (MPSC)
Regulatory environment
Number of filings required related to the Tax Cuts and Jobs Act of 2017 | Filings			3
Number of days between the first and second filing related to the impact on base rates for the change in tax expense associated with the Tax Cuts and Jobs Act of 2017 | day			30
MERC | Minnesota Public Utilities Commission (MPUC) | 2018 rates | Natural gas rates
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%
Interim rate increase		$ 7.0			$ 9.5
Interim rate increase, (as a percent)		2.81%			3.78%
Interim rate reduction		$ 2.5
Interim rates return on equity (as a percent)		9.10%
Interim rates common equity component average (as a percent)		50.90%
UMERC
Regulatory environment
Capacity of natural gas-fired generation facility (in megawatts) | MW						180
UMERC | Michigan Public Service Commission (MPSC)
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						$ 266.0
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%
Utility Operations | Tax Cuts And Jobs Act Of 2017
Regulatory environment
Change in deferred taxes from tax legislation				$ 2,450.0