SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment information
Number of reportable segments \| segment			6
Operating revenues	$ 1,643.7	$ 1,657.5	$ 5,602.7	$ 5,593.5
Other operation and maintenance	553.1	473.1	1,602.7	1,457.4
Depreciation and amortization	212.8	201.2	628.1	593.5
Operating income (loss)	302.7	392.2	1,178.6	1,369.1
Equity in earnings of transmission affiliates	33.7	39.2	95.2	122.9
Interest expense	112.0	103.8	327.2	310.4
Wisconsin
Segment information
Operating revenues	1,388.7		4,303.3
Illinois
Segment information
Operating revenues	197.9		973.2
Other States
Segment information
Operating revenues	50.2		292.5
Electric Transmission
Segment information
Operating revenues	0.0		0.0
Other operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliates	33.7	39.2	95.2	122.9
Interest expense	0.0	0.0	0.0	0.0
Non-Utility Energy Infrastructure
Segment information
Operating revenues	115.2		350.3
Other operation and maintenance	3.0	1.5	9.2	4.6
Depreciation and amortization	19.1	18.2	55.7	53.1
Operating income (loss)	91.6	103.4	277.0	299.5
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	15.9	16.2	48.0	46.7
Corporate and Other
Segment information
Operating revenues	2.4		6.9
Other operation and maintenance	(4.4)	(0.4)	(2.5)	3.5
Depreciation and amortization	7.1	6.3	22.3	18.3
Operating income (loss)	(0.4)	0.0	(12.3)	(10.1)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	32.9	25.4	91.2	81.3
Reconciling Eliminations
Segment information
Other operation and maintenance	(98.0)	(109.0)	(295.2)	(330.6)
Depreciation and amortization	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	$ (1.3)	0.4	$ (3.2)	(2.5)
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	$ (0.9)	(1.7)	$ (2.6)	(8.5)
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	80.00%		80.00%
Total utility revenues
Segment information
Other operation and maintenance	$ 652.5	581.0	$ 1,891.2	1,779.9
Depreciation and amortization	186.6	176.7	550.1	522.1
Operating income (loss)	211.5	288.8	913.9	1,079.7
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	64.5	61.8	191.2	184.9
Total utility revenues \| Wisconsin
Segment information
Other operation and maintenance	525.0	460.5	1,495.9	1,385.9
Depreciation and amortization	137.2	131.5	406.9	391.1
Operating income (loss)	201.4	277.5	670.2	829.6
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	49.6	48.5	147.5	145.4
Total utility revenues \| Illinois
Segment information
Other operation and maintenance	104.5	99.0	320.8	321.0
Depreciation and amortization	43.0	38.9	125.7	112.6
Operating income (loss)	15.5	14.3	204.8	214.9
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	12.8	11.0	37.4	33.0
Total utility revenues \| Other States
Segment information
Other operation and maintenance	23.0	21.5	74.5	73.0
Depreciation and amortization	6.4	6.3	17.5	18.4
Operating income (loss)	(5.4)	(3.0)	38.9	35.2
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	2.1	2.3	6.3	6.5
External Revenues
Segment information
Operating revenues	1,643.7	1,657.5	5,602.7	5,593.5
External Revenues \| Electric Transmission
Segment information
Operating revenues	0.0	0.0	0.0	0.0
External Revenues \| Non-Utility Energy Infrastructure
Segment information
Operating revenues	4.5	13.6	26.8	26.1
External Revenues \| Corporate and Other
Segment information
Operating revenues	2.4	5.6	6.9	11.7
External Revenues \| Reconciling Eliminations
Segment information
Operating revenues	0.0	0.0	0.0	0.0
External Revenues \| Total utility revenues
Segment information
Operating revenues	1,636.8	1,638.3	5,569.0	5,555.7
External Revenues \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	1,388.7	1,401.3	4,303.3	4,316.6
External Revenues \| Total utility revenues \| Illinois
Segment information
Operating revenues	197.9	187.2	973.2	965.7
External Revenues \| Total utility revenues \| Other States
Segment information
Operating revenues	50.2	49.8	292.5	273.4
Intersegment Transactions [Member]
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Transactions [Member] \| Electric Transmission
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Transactions [Member] \| Non-Utility Energy Infrastructure
Segment information
Operating revenues	110.7	111.6	323.5	333.2
Intersegment Transactions [Member] \| Corporate and Other
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Transactions [Member] \| Reconciling Eliminations
Segment information
Operating revenues	(110.7)	(111.6)	(323.5)	(333.2)
Intersegment Transactions [Member] \| Total utility revenues
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Illinois
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Other States
Segment information
Operating revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0

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