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Regulatory Environment (Details) $ in Millions	1 Months Ended																12 Months Ended
	Feb. 28, 2018 USD ($)	Dec. 31, 2017 USD ($) Assurance	Nov. 30, 2017 USD ($)	Oct. 31, 2017 USD ($) wind_turbines utility MW	Sep. 30, 2017 USD ($) utility	Jan. 31, 2017	Oct. 31, 2016 USD ($)	Aug. 31, 2016 MW	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015	Apr. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 31, 2014 USD ($)	Dec. 31, 2017 USD ($) Assurance	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory environment
AFUDC																	$ 4.9	$ 10.9	$ 8.6
Percent fuel costs can vary from the rate case approved costs before deferral is required																	2.00%
WE | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 Rates
Regulatory environment
Approved return on equity (as a percent)					10.20%
WE | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Electric rates
Regulatory environment
Approved rate increase (decrease)															$ 26.6
Approved rate increase (decrease), percentage															0.90%
WE | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Natural gas rates
Regulatory environment
Approved rate increase (decrease)															$ 0.0
WE | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Steam rates | Downtown Milwaukee (Valley) steam customers
Regulatory environment
Approved rate increase (decrease)															0.0
WE | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Steam rates | Milwaukee County steam customers
Regulatory environment
Approved rate increase (decrease)															$ 0.0
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates
Regulatory environment
Approved return on equity (as a percent)															10.20%
Approved common equity component average (as a percent)															51.00%
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Electric rates
Regulatory environment
Refund related to prior fuel costs and the remainder of the proceeds of a Treasury Grant															$ 26.6
Percent fuel costs can vary from the rate case approved costs before deferral is required															2.00%
SSR revenues															$ 90.7
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Electric rates | Non-fuel costs
Regulatory environment
Approved rate increase (decrease)															$ 2.7
Approved rate increase (decrease), percentage															0.10%
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Electric rates | Fuel costs
Regulatory environment
Approved rate increase (decrease)															$ (13.9)
Approved rate increase (decrease), percentage															(0.50%)
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Natural gas rates
Regulatory environment
Approved rate increase (decrease)															$ (10.7)
Approved rate increase (decrease), percentage															(2.40%)
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Steam rates | Downtown Milwaukee (Valley) steam customers
Regulatory environment
Approved rate increase (decrease)															$ 0.5
Approved rate increase (decrease), percentage															2.00%
WE | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Steam rates | Milwaukee County steam customers
Regulatory environment
Approved rate increase (decrease)															$ 1.2
Approved rate increase (decrease), percentage															7.30%
WG | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 Rates
Regulatory environment
Approved return on equity (as a percent)					10.30%
WG | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Natural gas rates
Regulatory environment
Approved rate increase (decrease)															$ 21.4
Approved rate increase (decrease), percentage															3.20%
WG | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Natural gas rates
Regulatory environment
Approved return on equity (as a percent)															10.30%
Approved rate increase (decrease)															$ 17.1
Approved rate increase (decrease), percentage															2.60%
Approved common equity component average (as a percent)															49.50%
WPS | Forward Wind Energy Center
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 Forward 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
WPS | Public Service Commission of Wisconsin (PSCW) | 2016 Rates
Regulatory environment
Approved return on equity (as a percent)										10.00%
Approved common equity component average (as a percent)										51.00%
WPS | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Electric rates
Regulatory environment
Authorized revenue requirement for the ReACT project									$ 275.0	$ 275.0
Approved rate increase (decrease)										$ (7.9)
Approved rate increase (decrease), percentage										(0.80%)
Percent fuel costs can vary from the rate case approved costs before deferral is required										2.00%
WPS | Public Service Commission of Wisconsin (PSCW) | 2016 Rates | Natural gas rates
Regulatory environment
Approved rate increase (decrease)										$ (6.2)
Approved rate increase (decrease), percentage										(2.10%)
WPS | Public Service Commission of Wisconsin (PSCW) | 2015 Rates
Regulatory environment
Approved return on equity (as a percent)															10.20%
Approved common equity component average (as a percent)															50.28%
WPS | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Electric rates
Regulatory environment
Approved rate increase (decrease)															$ 24.6
Percent fuel costs can vary from the rate case approved costs before deferral is required															2.00%
Increase in cost of fuel for electric generation															$ 42.0
Year-over-year positive (negative) change in decoupling refunded to or collected from customers															9.0
WPS | Public Service Commission of Wisconsin (PSCW) | 2015 Rates | Natural gas rates
Regulatory environment
Approved rate increase (decrease)															(15.4)
Year-over-year positive (negative) change in decoupling refunded to or collected from customers															$ (16.0)
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 and NSG | Illinois Commerce Commission (ICC) | Natural gas rates
Regulatory environment
Period of base rate freeze											2 years
PGL | Illinois Commerce Commission (ICC)
Regulatory environment
Amount of assurance that costs incurred under QIP rider are recoverable | Assurance		0															0
PGL | Illinois Commerce Commission (ICC) | Subsequent event
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
PGL | Illinois Commerce Commission (ICC) | 2015 Rates | Natural gas rates
Regulatory environment
Approved return on equity (as a percent)														9.05%
Approved rate increase (decrease)														$ 74.8
Approved common equity component average (as a percent)														50.33%
Amended approved rate increase (decrease)													$ 71.1
NSG | Illinois Commerce Commission (ICC) | 2015 Rates | Natural gas rates
Regulatory environment
Approved return on equity (as a percent)														9.05%
Approved rate increase (decrease)														$ 3.7
Approved common equity component average (as a percent)														50.48%
Amended approved rate increase (decrease)													$ 3.5
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			$ 9.5
Interim rate increase (as a percent)			3.78%
Interim rates return on equity (as a percent)			9.11%
Interim rates 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 (decrease)							$ 6.8
Approved rate increase (decrease), percentage							3.00%
Approved common equity component average (as a percent)							50.32%
Number of years decoupling mechanism authorized for use							3 years
Interim rates refunded to customers		$ 4.1															$ 4.1
MERC | Minnesota Public Utilities Commission (MPUC) | 2015 Rates | Natural gas rates
Regulatory environment
Approved return on equity (as a percent)																9.35%
Approved rate increase (decrease)																$ 7.6
Approved common equity component average (as a percent)																50.31%
Interim rates refunded to customers										$ 4.7									$ 4.7
Annual cap for decoupling mechanism (as a percent of rate case approved distribution revenues)																10.00%
MGU | Michigan Public Service Commission (MPSC) | 2016 Rates | Natural gas rates
Regulatory environment
Approved return on equity (as a percent)										9.90%
Approved rate increase (decrease)										$ 3.4
Approved rate increase (decrease), percentage										2.40%
Approved common equity component average (as a percent)										52.00%
UMERC
Regulatory environment
Term of electric power purchase agreement (in years)								20 years
Capacity of natural gas-fired generation facility (in megawatts) | MW				180				180
Estimated cost of constructing 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%
UMERC | Michigan Public Service Commission (MPSC) | 2015 Rates | Electric rates
Regulatory environment
Approved return on equity (as a percent)												10.20%
Approved rate increase (decrease)												$ 4.0
Approved common equity component average (as a percent)												50.48%
Period of rate implementation												3 years
Utility operations
Regulatory environment
Change in deferred income taxes from tax legislation																	$ 2,450.0
Utility operations | Tax Cuts and Jobs Act of 2017
Regulatory environment
Change in deferred income taxes from tax legislation		$ 2,450.0

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Fuel costs incurred that are directly related to goods produced and sold and services rendered during the reporting period.

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Amount of disallowed costs for rate making purposes for recently completed plants in regulated industries.

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Amount of assurance that the costs incurred under PGL's Qualifying Infrastructure Plant rider will be recoverable.

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The annual cap (as a percent) for a decoupling mechanism.

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Total capacity, in megawatts, of the Forward Wind Energy Center.

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The cost of constructing a power plant.

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Term (in years) of an electric power purchase agreement with the Tilden Mining Company.

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The rate base freeze period imposed by the Illinois Commerce Commission as a result of the acquisition of Integrys Energy Group, Inc.

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The number of utilities, in addition to Wisconsin Public Service Corporation, that enetered into an agreement to purchase the Forward Wind Energy Center.

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Number of utilities that agreed to employ an earnings sharing mechanism.

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Percentage of the company's Wisconsin natural gas storage needs provided by the natural gas storage facility.

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The system support resource revenues received from Midcontinent Independent System Operator, Inc.

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Number of wInd turbines at Forward Wind Energy Center.

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