SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment information
Number of reportable segments \| segment			6
Operating revenues	$ 1,608.0	$ 1,643.7	$ 5,575.6	$ 5,602.7
Other operation and maintenance	529.2	553.1	1,583.4	1,602.7
Depreciation and amortization	233.8	212.8	690.1	628.1
Operating income (loss)	310.9	302.7	1,168.3	1,178.6
Equity in earnings of transmission affiliates	38.7	33.7	111.7	95.2
Interest expense	125.8	112.0	374.3	327.2
Wisconsin
Segment information
Operating revenues	1,339.3	1,388.7	4,226.0	4,303.3
Illinois
Segment information
Operating revenues	198.0	197.9	977.4	973.2
Other States
Segment information
Operating revenues	48.9	50.2	302.9	292.5
Electric Transmission
Segment information
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	38.7	33.7	111.7	95.2
Interest expense	3.0	0.0	$ 8.3	0.0
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities			33.00%
Operating revenues	119.4	115.2	$ 370.5	350.3
Other operation and maintenance	3.9	3.0	14.5	9.2
Depreciation and amortization	23.3	19.1	68.8	55.7
Operating income (loss)	90.3	91.6	274.3	277.0
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	15.5	15.9	46.7	48.0
Corporate and Other
Segment information
Operating revenues	1.3	2.4	3.9	6.9
Other operation and maintenance	1.3	(4.4)	2.1	(2.5)
Depreciation and amortization	6.0	7.1	18.4	22.3
Operating income (loss)	(6.0)	(0.4)	(17.0)	(12.3)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	35.9	32.9	107.5	91.2
Reconciling Eliminations
Segment information
Other operation and maintenance	0.0	(98.0)	0.0	(295.2)
Depreciation and amortization	(3.6)	0.0	(11.8)	0.0
Operating income (loss)	(86.8)	0.0	(261.0)	0.0
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	$ (88.5)	(1.3)	$ (267.1)	(3.2)
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	$ 0.4	(0.9)	$ (0.5)	(2.6)
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 Wind \| 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%
Total utility revenues
Segment information
Other operation and maintenance	$ 524.0	652.5	$ 1,566.8	1,891.2
Depreciation and amortization	208.1	186.6	614.7	550.1
Operating income (loss)	313.4	211.5	1,172.0	913.9
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	159.9	64.5	478.9	191.2
Total utility revenues \| Wisconsin
Segment information
Other operation and maintenance	400.2	525.0	1,156.8	1,495.9
Depreciation and amortization	155.6	137.2	459.5	406.9
Operating income (loss)	290.8	201.4	922.8	670.2
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	142.9	49.6	429.0	147.5
Total utility revenues \| Illinois
Segment information
Other operation and maintenance	101.8	104.5	337.0	320.8
Depreciation and amortization	45.7	43.0	135.2	125.7
Operating income (loss)	24.8	15.5	205.3	204.8
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	14.7	12.8	43.4	37.4
Total utility revenues \| Other States
Segment information
Other operation and maintenance	22.0	23.0	73.0	74.5
Depreciation and amortization	6.8	6.4	20.0	17.5
Operating income (loss)	(2.2)	(5.4)	43.9	38.9
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	2.3	2.1	6.5	6.3
External Revenues
Segment information
Operating revenues	1,608.0	1,643.7	5,575.6	5,602.7
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	20.5	4.5	65.4	26.8
External Revenues \| Corporate and Other
Segment information
Operating revenues	1.3	2.4	3.9	6.9
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	1,586.2	1,636.8	5,506.3	5,569.0
External Revenues \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	1,339.3	1,388.7	4,226.0	4,303.3
External Revenues \| Total utility revenues \| Illinois
Segment information
Operating revenues	198.0	197.9	977.4	973.2
External Revenues \| Total utility revenues \| Other States
Segment information
Operating revenues	48.9	50.2	302.9	292.5
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	98.9	110.7	305.1	323.5
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	(98.9)	(110.7)	(305.1)	(323.5)
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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