REGULATORY ENVIRONMENT (Details) $ in Millions	1 Months Ended							3 Months Ended
	Mar. 31, 2019 USD ($) Assurance company	Dec. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Feb. 28, 2018 Filings	Nov. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) utility	Mar. 31, 2019 USD ($) Assurance	Mar. 31, 2018 USD ($)	May 31, 2018 USD ($) solar_projects MW
Regulatory environment
Capital investments related to gas utility infrastructure								$ 358.8	$ 439.6
Operation and maintenance expenses related to gas utility infrastructure								550.6	$ 511.9
WE | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Electric rates
Regulatory environment
Requested rate increase	$ 83.0
Requested rate increase (as a percent)	2.90%
WE | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Natural gas rates
Regulatory environment
Requested rate increase	$ 15.0
Requested rate increase (as a percent)	3.90%
WE | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Steam rates
Regulatory environment
Requested rate increase	$ 1.0
Requested rate increase (as a percent)	4.50%
WE | Public Service Commission of Wisconsin (PSCW) | 2021 rates | Electric rates
Regulatory environment
Requested rate increase	$ 83.0
Requested rate increase (as a percent)	2.90%
WE | Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates
Regulatory environment
Requested return on equity (as a percent)	10.35%
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								$ 0.0
WG | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Natural gas rates
Regulatory environment
Requested rate increase	$ 11.0
Requested rate increase (as a percent)	1.80%
WG | Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates
Regulatory environment
Requested return on equity (as a percent)	10.30%
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) | Badger Hollow Solar Farm and Two Creeks Solar Project
Regulatory environment
Number of solar projects for which approval has been requested | solar_projects										2
Total output of two solar projects owned by WPS (in megawatts) | MW										200
WPS share of cost for certain solar projects										$ 260.0
WPS | Public Service Commission of Wisconsin (PSCW) | Two Creeks Solar Project
Regulatory environment
Solar project output owned by WPS (in megawatts) | MW										100
WPS | Public Service Commission of Wisconsin (PSCW) | Badger Hollow Solar Farm
Regulatory environment
Solar project output owned by WPS (in megawatts) | MW										100
WPS | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Electric rates
Regulatory environment
Requested rate increase	$ 49.0
Requested rate increase (as a percent)	4.90%
WPS | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Natural gas rates
Regulatory environment
Requested rate increase	$ 7.0
Requested rate increase (as a percent)	2.40%
WPS | Public Service Commission of Wisconsin (PSCW) | 2021 rates | Electric rates
Regulatory environment
Requested rate increase	$ 49.0
Requested rate increase (as a percent)	4.90%
WPS | Public Service Commission of Wisconsin (PSCW) | 2021 rates | Natural gas rates
Regulatory environment
Requested rate increase	$ 7.0
Requested rate increase (as a percent)	2.40%
WPS | Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates
Regulatory environment
Requested return on equity (as a percent)	10.35%
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) | 2020 and 2021 rates
Regulatory environment
Number of companies filing rate increase request | company	3
Requested common equity component average (as a percent)	52.00%
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							0
MERC | Minnesota Public Utilities Commission (MPUC) | Gas utility infrastructure costs
Regulatory environment
Capital investments related to gas utility infrastructure			$ 12.0
Operation and maintenance expenses related to gas utility infrastructure			$ 3.0
MERC | Minnesota Public Utilities Commission (MPUC) | 2018 rates | Natural gas rates
Regulatory environment
Change in regulatory liabilities								$ 6.7
Approved return on equity (as a percent)		9.70%
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)				9.11%
Interim rates common equity component average (as a percent)				50.90%
Approved rate increase		$ 3.1
Approved rate increase (as a percent)		1.26%
Approved common equity component average (as a percent)		50.90%
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
Tax Cuts and Jobs Act of 2017 | WE | Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates | Electric rates
Regulatory environment
Change in regulatory liabilities	$ 111.0
Tax Cuts and Jobs Act of 2017 | WPS | Public Service Commission of Wisconsin (PSCW) | 2020 rates | Natural gas rates
Regulatory environment
Change in regulatory liabilities	7.0
Tax Cuts and Jobs Act of 2017 | WPS | Public Service Commission of Wisconsin (PSCW) | 2020 and 2021 rates | Electric rates
Regulatory environment
Change in regulatory liabilities	$ 40.0