Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment information
Number of reportable segments | segment									6
Revenues	$ 2,055.0	$ 1,657.5	$ 1,631.5	$ 2,304.5	$ 1,963.0	$ 1,712.5	$ 1,602.0	$ 2,194.8	$ 7,648.5	$ 7,472.3	$ 5,926.1
Other operation and maintenance									2,047.0	2,185.5	1,709.3
Depreciation and amortization									798.6	762.6	561.8
Operating income (loss)	412.1	$ 393.6	$ 362.2	$ 617.3	361.7	$ 399.0	$ 332.1	$ 589.3	1,785.2	1,682.1	1,250.5
Equity in earnings of transmission affiliates									154.3	146.5	96.1
Interest expense									415.7	402.7	331.4
Capital expenditures									1,959.5	1,423.7	1,266.2
Total assets	31,590.5				30,123.2				31,590.5	30,123.2	29,355.2
Intersegment revenues
Segment information
Revenues									0.0	0.0	0.0
Electric transmission
Segment information
Revenues									0.0	0.0	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									154.3	146.5	96.1
Interest expense									0.0	0.0	0.0
Capital expenditures									0.0	0.0	0.0
Total assets	1,593.4				1,476.9				1,593.4	1,476.9	1,381.0
Electric transmission | Intersegment revenues
Segment information
Revenues									0.0	0.0	0.0
Non-Utility Energy Infrastructure
Segment information
Revenues									38.9	24.9	40.0
Other operation and maintenance									7.3	4.3	4.3
Depreciation and amortization									71.4	68.3	67.5
Operating income (loss)									400.5	375.6	373.4
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									62.8	62.1	63.4
Capital expenditures									35.4	62.3	53.4
Total assets	2,992.8				2,777.1				2,992.8	2,777.1	2,779.0
Non-Utility Energy Infrastructure | Intersegment revenues
Segment information
Revenues									446.3	423.3	405.2
Corporate and other
Segment information
Revenues									13.7	23.3	47.3
Other operation and maintenance									(4.1)	(15.8)	103.7
Depreciation and amortization									25.9	42.6	12.4
Operating income (loss)									(8.4)	(10.0)	(91.2)
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									107.3	120.9	91.0
Capital expenditures									152.1	97.8	33.4
Total assets	953.6				778.0				953.6	778.0	1,132.5
Corporate and other | Intersegment revenues
Segment information
Revenues									0.0	0.0	0.0
Reconciling eliminations
Segment information
Revenues									0.0	0.0	0.0
Other operation and maintenance									(441.1)	(423.6)	(409.3)
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									(1.8)	(8.6)	(5.1)
Capital expenditures									0.0	0.0	0.0
Total assets	(3,398.9)				(3,349.2)				(3,398.9)	(3,349.2)	(3,431.7)
Reconciling eliminations | WE
Segment information
Total assets	2,038.1				2,029.5				2,038.1	2,029.5	2,105.3
Reconciling eliminations | Intersegment revenues
Segment information
Revenues									(446.3)	(423.6)	(410.2)
Public Utility
Segment information
Revenues									7,595.9	7,424.1	5,838.8
Other operation and maintenance									2,484.9	2,620.6	2,010.6
Depreciation and amortization									701.3	651.7	481.9
Operating income (loss)									1,393.1	1,316.5	968.3
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									247.4	228.3	182.1
Capital expenditures									1,772.0	1,263.6	1,179.4
Total assets	29,449.6				28,440.4				29,449.6	28,440.4	27,494.4
Public Utility | Intersegment revenues
Segment information
Revenues									0.0	0.3	5.0
Public Utility | Wisconsin
Segment information
Revenues									5,829.2	5,805.4	5,186.1
Other operation and maintenance									1,912.5	2,025.4	1,741.0
Depreciation and amortization									523.9	496.6	408.6
Operating income (loss)									1,065.9	1,027.0	884.2
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									193.7	180.9	157.1
Capital expenditures									1,152.3	910.9	950.3
Total assets	22,237.1				21,730.7				22,237.1	21,730.7	21,113.5
Public Utility | Wisconsin | Intersegment revenues
Segment information
Revenues									0.0	0.3	5.0
Public Utility | Illinois
Segment information
Revenues									1,355.5	1,242.2	503.4
Other operation and maintenance									471.1	485.1	219.6
Depreciation and amortization									152.6	134.0	63.3
Operating income (loss)									273.0	239.6	78.1
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									45.0	38.9	19.9
Capital expenditures									545.2	293.2	194.4
Total assets	6,144.7				5,714.6				6,144.7	5,714.6	5,462.9
Public Utility | Illinois | Intersegment revenues
Segment information
Revenues									0.0	0.0	0.0
Public Utility | Other States
Segment information
Revenues									411.2	376.5	149.3
Other operation and maintenance									101.3	110.1	50.0
Depreciation and amortization									24.8	21.1	10.0
Operating income (loss)									54.2	49.9	6.0
Equity in earnings of transmission affiliates									0.0	0.0	0.0
Interest expense									8.7	8.5	5.1
Capital expenditures									74.5	59.5	34.7
Total assets	$ 1,067.8				$ 995.1				1,067.8	995.1	918.0
Public Utility | Other States | Intersegment revenues
Segment information
Revenues									$ 0.0	0.0	0.0
ATC
Segment information
Equity method investment, ownership interest (as a percent)	60.00%								60.00%
Equity in earnings of transmission affiliates									$ 166.0	$ 146.5	$ 96.1