SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment information
Number of reportable segments \| segment	6
Operating revenues	$ 2,108.6	$ 2,377.4
Other operation and maintenance	455.7	550.6
Depreciation and amortization	239.1	226.4
Operating income (loss)	626.6	542.8
Equity in earnings of transmission affiliates	39.8	36.1
Interest expense	129.4	124.4
Wisconsin
Segment information
Operating revenues	1,498.9	1,633.4
Illinois
Segment information
Operating revenues	447.6	536.5
Other States
Segment information
Operating revenues	146.4	185.2
Electric Transmission
Segment information
Other operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Operating income (loss)	0.0	0.0
Equity in earnings of transmission affiliates	39.8	36.1
Interest expense	$ 4.8	2.6
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities	33.00%
Operating revenues	$ 129.6	127.8
Other operation and maintenance	5.2	3.8
Depreciation and amortization	24.5	22.6
Operating income (loss)	91.5	92.7
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	15.3	15.7
Corporate and Other
Segment information
Operating revenues	0.5	1.7
Other operation and maintenance	(1.6)	(1.0)
Depreciation and amortization	6.1	6.4
Operating income (loss)	(4.2)	(3.9)
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	35.1	35.1
Reconciling Eliminations
Segment information
Other operation and maintenance	(4.5)	(0.7)
Depreciation and amortization	(12.2)	(4.6)
Operating income (loss)	(86.5)	(87.2)
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	$ (87.1)	(89.5)
ATC \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	60.00%
ATC Holdco
Segment information
Equity method investment, ownership interest (as a percent)	75.00%
Equity in earnings of transmission affiliates	$ 0.2	(0.4)
ATC Holdco \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%
Bishop Hill III \| Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Bishop Hill III Wind Energy Center	90.00%
Coyote Ridge \| Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Coyote Ridge Wind, LLC	80.00%
Upstream \| Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Upstream Wind Energy Center	80.00%
Total utility revenues
Segment information
Other operation and maintenance	$ 456.6	548.5
Depreciation and amortization	220.7	202.0
Operating income (loss)	625.8	541.2
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	161.3	160.5
Total utility revenues \| Wisconsin
Segment information
Other operation and maintenance	330.8	392.7
Depreciation and amortization	165.4	151.0
Operating income (loss)	426.8	361.8
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	143.1	143.4
Total utility revenues \| Illinois
Segment information
Other operation and maintenance	104.1	128.2
Depreciation and amortization	47.5	44.5
Operating income (loss)	161.6	137.9
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	16.0	14.8
Total utility revenues \| Other States
Segment information
Other operation and maintenance	21.7	27.6
Depreciation and amortization	7.8	6.5
Operating income (loss)	37.4	41.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	2.2	2.3
External Revenues
Segment information
Operating revenues	2,108.6	2,377.4
External Revenues \| Electric Transmission
Segment information
Operating revenues	0.0	0.0
External Revenues \| Non-Utility Energy Infrastructure
Segment information
Operating revenues	15.2	20.6
External Revenues \| Corporate and Other
Segment information
Operating revenues	0.5	1.7
External Revenues \| Reconciling Eliminations
Segment information
Operating revenues	0.0	0.0
External Revenues \| Total utility revenues
Segment information
Operating revenues	2,092.9	2,355.1
External Revenues \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	1,498.9	1,633.4
External Revenues \| Total utility revenues \| Illinois
Segment information
Operating revenues	447.6	536.5
External Revenues \| Total utility revenues \| Other States
Segment information
Operating revenues	146.4	185.2
Intersegment Transactions [Member]
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Electric Transmission
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Non-Utility Energy Infrastructure
Segment information
Operating revenues	114.4	107.2
Intersegment Transactions [Member] \| Corporate and Other
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Reconciling Eliminations
Segment information
Operating revenues	(114.4)	(107.2)
Intersegment Transactions [Member] \| Total utility revenues
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Illinois
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Other States
Segment information
Operating revenues	$ 0.0	$ 0.0

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