Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment information
Number of reportable segments | segment									6
Revenues	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 2,055.0	$ 1,657.5	$ 1,631.5	$ 2,304.5	$ 7,679.5	$ 7,648.5	$ 7,472.3
Other operation and maintenance									2,270.5	2,056.1	2,171.3
Depreciation and amortization									845.8	798.6	762.6
Operating income (loss)	289.8	$ 302.7	$ 330.8	$ 545.1	407.0	$ 392.2	$ 362.2	$ 614.7	1,468.4	1,776.1	1,696.3
Equity in earnings of transmission affiliates									136.7	154.3	146.5
Interest expense									445.1	415.7	402.7
Capital expenditures and asset acquisitions									2,417.1	1,959.5	1,423.7
Total assets	33,475.8				31,590.5				33,475.8	31,590.5	30,123.2
Wisconsin
Segment information
Revenues									5,794.7
Illinois
Segment information
Revenues									1,400.0
Other States
Segment information
Revenues									438.2
Electric transmission
Segment information
Revenues									0.0
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									136.7	154.3	146.5
Interest expense									0.3	0.0	0.0
Capital expenditures and asset acquisitions									0.0	0.0	0.0
Total assets	1,665.3				1,593.4				1,665.3	1,593.4	1,476.9
Non-Utility Energy Infrastructure
Segment information
Revenues									468.4
Other operation and maintenance									12.6	7.3	4.3
Depreciation and amortization									75.7	71.4	68.3
Operating income (loss)									365.8	400.5	375.6
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									63.7	62.8	62.1
Capital expenditures and asset acquisitions									260.6	35.4	62.3
Total assets	3,227.2				2,992.8				3,227.2	2,992.8	2,777.1
Corporate and other
Segment information
Revenues									8.7
Other operation and maintenance									1.8	1.4	(16.4)
Depreciation and amortization									29.1	25.9	42.6
Operating income (loss)									(22.2)	(13.9)	(9.4)
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									125.8	107.3	120.9
Capital expenditures and asset acquisitions									39.7	152.1	97.8
Total assets	959.6				953.6				959.6	953.6	778.0
Reconciling eliminations
Segment information
Other operation and maintenance									(393.3)	(441.1)	(423.6)
Depreciation and amortization									0.0	0.0	0.0
Operating income (loss)									0.0	0.0	0.0
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									(5.3)	(1.8)	(8.6)
Capital expenditures and asset acquisitions									0.0	0.0	0.0
Total assets	(3,414.5)				(3,398.9)				(3,414.5)	(3,398.9)	(3,349.2)
Reconciling eliminations | WE
Segment information
Total assets	$ 1,968.5				2,038.1				$ 1,968.5	2,038.1	2,029.5
ATC
Segment information
Equity method investment, ownership interest (as a percent)	60.00%								60.00%
Equity in earnings of transmission affiliates									$ 139.6	166.0	146.5
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									$ (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
Segment information
WEC's ownership interest in Bishop Hill III Wind Energy Center	90.00%								90.00%
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
Segment information
WEC's ownership interest in Coyote Ridge Wind, LLC	80.00%								80.00%
Coyote Ridge | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Coyote Ridge Wind, LLC	80.00%								80.00%
Public Utilities [Member]
Segment information
Other operation and maintenance									$ 2,649.4	2,488.5	2,607.0
Depreciation and amortization									741.0	701.3	651.7
Operating income (loss)									1,124.8	1,389.5	1,330.1
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									260.6	247.4	228.3
Capital expenditures and asset acquisitions									2,116.8	1,772.0	1,263.6
Total assets	$ 31,038.2				29,449.6				31,038.2	29,449.6	28,440.4
Public Utilities [Member] | Wisconsin
Segment information
Other operation and maintenance									2,076.1	1,923.2	2,034.6
Depreciation and amortization									546.6	523.9	496.6
Operating income (loss)									800.2	1,055.2	1,017.8
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									200.7	193.7	180.9
Capital expenditures and asset acquisitions									1,466.1	1,152.3	910.9
Total assets	23,407.0				22,237.1				23,407.0	22,237.1	21,730.7
Public Utilities [Member] | Illinois
Segment information
Other operation and maintenance									472.3	464.2	463.6
Depreciation and amortization									170.3	152.6	134.0
Operating income (loss)									255.8	279.9	261.1
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									51.2	45.0	38.9
Capital expenditures and asset acquisitions									547.1	545.2	293.2
Total assets	6,483.3				6,144.7				6,483.3	6,144.7	5,714.6
Public Utilities [Member] | Other States
Segment information
Other operation and maintenance									101.0	101.1	108.8
Depreciation and amortization									24.1	24.8	21.1
Operating income (loss)									68.8	54.4	51.2
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									8.7	8.7	8.5
Capital expenditures and asset acquisitions									103.6	74.5	59.5
Total assets	$ 1,147.9				$ 1,067.8				1,147.9	1,067.8	995.1
External Revenues | Electric transmission
Segment information
Revenues									0.0	0.0	0.0
External Revenues | Non-Utility Energy Infrastructure
Segment information
Revenues									37.9	38.9	24.9
External Revenues | Corporate and other
Segment information
Revenues									8.7	13.7	23.3
External Revenues | Reconciling eliminations
Segment information
Revenues									0.0	0.0	0.0
External Revenues | Public Utilities [Member]
Segment information
Revenues									7,632.9	7,595.9	7,424.1
External Revenues | Public Utilities [Member] | Wisconsin
Segment information
Revenues									5,794.7	5,829.2	5,805.4
External Revenues | Public Utilities [Member] | Illinois
Segment information
Revenues									1,400.0	1,355.5	1,242.2
External Revenues | Public Utilities [Member] | Other States
Segment information
Revenues									438.2	411.2	376.5
Intersegment Transactions [Member]
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions [Member] | Electric transmission
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions [Member] | Non-Utility Energy Infrastructure
Segment information
Revenues									430.5	446.3	423.3
Intersegment Transactions [Member] | Corporate and other
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions [Member] | Reconciling eliminations
Segment information
Revenues									(430.5)	(446.3)	(423.6)
Intersegment Transactions [Member] | Public Utilities [Member]
Segment information
Revenues									0.0	0.0	0.3
Intersegment Transactions [Member] | Public Utilities [Member] | Wisconsin
Segment information
Revenues									0.0	0.0	0.3
Intersegment Transactions [Member] | Public Utilities [Member] | Illinois
Segment information
Revenues									0.0	0.0	0.0
Intersegment Transactions [Member] | Public Utilities [Member] | Other States
Segment information
Revenues									$ 0.0	$ 0.0	$ 0.0