Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment information
Number of reportable segments | segment									6
Revenues	$ 1,947.5	$ 1,608.0	$ 1,590.2	$ 2,377.4	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 7,523.1	$ 7,679.5	$ 7,648.5
Other operation and maintenance									2,184.8	2,270.5	2,056.1
Depreciation and amortization									926.3	845.8	798.6
Operating income (loss)	363.1	$ 310.9	$ 314.6	$ 542.8	289.8	$ 302.7	$ 330.8	$ 545.1	1,531.4	1,468.4	1,776.1
Equity in earnings of transmission affiliates									127.6	136.7	154.3
Interest expense									501.5	445.1	415.7
Capital expenditures and asset acquisitions									2,529.0	2,417.1	1,959.5
Total assets	34,951.8				33,475.8				34,951.8	33,475.8	31,590.5
Wisconsin
Segment information
Revenues									5,647.1	5,794.7
Illinois
Segment information
Revenues									1,357.1	1,400.0
Other States
Segment information
Revenues									426.0	438.2
Electric transmission
Segment information
Other operation and maintenance									0.0	0.0	0.0
Depreciation and amortization									0.0	0.0	0.0
Operating income (loss)									0.0	0.0	0.0
Equity in earnings of transmission affiliates									127.6	136.7	154.3
Interest expense									13.1	0.3	0.0
Capital expenditures and asset acquisitions									0.0	0.0	0.0
Total assets	1,723.1				1,665.3				$ 1,723.1	1,665.3	1,593.4
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities									33.00%
Revenues									$ 495.9	468.4
Other operation and maintenance									19.7	12.6	7.3
Depreciation and amortization									92.0	75.7	71.4
Operating income (loss)									366.6	365.8	400.5
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									62.1	63.7	62.8
Capital expenditures and asset acquisitions									389.9	260.6	35.4
Total assets	3,654.1				3,227.2				3,654.1	3,227.2	2,992.8
Corporate and other
Segment information
Revenues									4.4	8.7
Other operation and maintenance									14.0	1.8	1.4
Depreciation and amortization									24.3	29.1	25.9
Operating income (loss)									(34.4)	(22.2)	(13.9)
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									140.9	125.8	107.3
Capital expenditures and asset acquisitions									26.5	39.7	152.1
Total assets	814.0				959.6				814.0	959.6	953.6
Reconciling eliminations
Segment information
Other operation and maintenance									0.2	(393.3)	(441.1)
Depreciation and amortization									(15.8)	0.0	0.0
Operating income (loss)									(347.6)	0.0	0.0
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									(354.1)	(5.3)	(1.8)
Capital expenditures and asset acquisitions									0.0	0.0	0.0
Total assets	(3,344.5)				(3,414.5)				(3,344.5)	(3,414.5)	(3,398.9)
Reconciling eliminations | WE
Segment information
Total assets	$ 1,896.7				1,968.5				$ 1,896.7	1,968.5	2,038.1
ATC
Segment information
Equity method investment, ownership interest (as a percent)	60.00%								60.00%
Equity in earnings of transmission affiliates									$ 132.8	139.6	166.0
ATC | Electric transmission
Segment information
Equity method investment, ownership interest (as a percent)	60.00%								60.00%
ATC HoldCo
Segment information
Equity method investment, ownership interest (as a percent)	75.00%								75.00%
Equity in earnings of transmission affiliates									$ (5.2)	(2.9)	(11.7)
ATC HoldCo | Electric transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%								75.00%
Bishop Hill III Wind Energy Center | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Bishop Hill III Wind Energy Center	90.00%								90.00%
Coyote Ridge | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Coyote Ridge Wind, LLC	80.00%								80.00%
Upstream | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Upstream Wind Energy Center	80.00%								80.00%
Public Utilities
Segment information
Other operation and maintenance									$ 2,150.9	2,649.4	2,488.5
Depreciation and amortization									825.8	741.0	701.3
Operating income (loss)									1,546.8	1,124.8	1,389.5
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									639.5	260.6	247.4
Capital expenditures and asset acquisitions									2,112.6	2,116.8	1,772.0
Total assets	$ 32,105.1				31,038.2				32,105.1	31,038.2	29,449.6
Public Utilities | Wisconsin
Segment information
Other operation and maintenance									1,591.3	2,076.1	1,923.2
Depreciation and amortization									617.0	546.6	523.9
Operating income (loss)									1,189.6	800.2	1,055.2
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									572.0	200.7	193.7
Capital expenditures and asset acquisitions									1,378.6	1,466.1	1,152.3
Total assets	23,934.8				23,407.0				23,934.8	23,407.0	22,237.1
Public Utilities | Illinois
Segment information
Other operation and maintenance									461.1	472.3	464.2
Depreciation and amortization									181.3	170.3	152.6
Operating income (loss)									291.9	255.8	279.9
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									59.0	51.2	45.0
Capital expenditures and asset acquisitions									624.9	547.1	545.2
Total assets	6,932.5				6,483.3				6,932.5	6,483.3	6,144.7
Public Utilities | Other States
Segment information
Other operation and maintenance									98.5	101.0	101.1
Depreciation and amortization									27.5	24.1	24.8
Operating income (loss)									65.3	68.8	54.4
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									8.5	8.7	8.7
Capital expenditures and asset acquisitions									109.1	103.6	74.5
Total assets	$ 1,237.8				$ 1,147.9				1,237.8	1,147.9	1,067.8
External Revenues | Electric transmission
Segment information
Revenues									0.0	0.0	0.0
External Revenues | Non-Utility Energy Infrastructure
Segment information
Revenues									88.5	37.9	38.9
External Revenues | Corporate and other
Segment information
Revenues									4.4	8.7	13.7
External Revenues | Reconciling eliminations
Segment information
Revenues									0.0	0.0	0.0
External Revenues | Public Utilities
Segment information
Revenues									7,430.2	7,632.9	7,595.9
External Revenues | Public Utilities | Wisconsin
Segment information
Revenues									5,647.1	5,794.7	5,829.2
External Revenues | Public Utilities | Illinois
Segment information
Revenues									1,357.1	1,400.0	1,355.5
External Revenues | Public Utilities | Other States
Segment information
Revenues									426.0	438.2	411.2
Intersegment Transactions
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions | Electric transmission
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions | Non-Utility Energy Infrastructure
Segment information
Revenues									407.4	430.5	446.3
Intersegment Transactions | Corporate and other
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions | Reconciling eliminations
Segment information
Revenues									(407.4)	(430.5)	(446.3)
Intersegment Transactions | Public Utilities
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions | Public Utilities | Wisconsin
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions | Public Utilities | Illinois
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions | Public Utilities | Other States
Segment information
Revenues									$ 0.0	$ 0.0	$ 0.0