SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Mar. 31, 2020 USD ($)
Segment information
Number of reportable segments | segment	6
Total operating revenues	$ 2,691.4	$ 2,108.6
Other operation and maintenance	479.9	455.7
Depreciation and amortization	261.4	239.1
Equity in earnings of transmission affiliates	42.6	39.8
Interest expense	119.5	129.4
Income tax expense (benefit)	74.9	90.0
Net income (loss)	510.3	452.6
Net income attributed to common shareholders	510.1	452.5
Reconciling Eliminations
Segment information
Other operation and maintenance	(1.6)	(4.5)
Depreciation and amortization	(14.3)	(12.2)
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	(85.7)	(87.1)
Income tax expense (benefit)	0.0	0.0
Net income (loss)	0.0	0.0
Net income attributed to common shareholders	0.0	0.0
Wisconsin
Segment information
Total operating revenues	1,731.7	1,498.9
Illinois
Segment information
Total operating revenues	703.4	447.6
Other States
Segment information
Total operating revenues	233.3	146.4
Electric Transmission
Segment information
Other operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Equity in earnings of transmission affiliates	42.6	39.8
Interest expense	4.9	4.8
Income tax expense (benefit)	9.8	9.9
Net income (loss)	28.0	25.0
Net income attributed to common shareholders	$ 28.0	25.0
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities	33.00%
Total operating revenues	$ 137.6	129.6
Other operation and maintenance	8.9	5.2
Depreciation and amortization	31.0	24.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	18.0	15.3
Income tax expense (benefit)	0.1	11.2
Net income (loss)	71.3	65.3
Net income attributed to common shareholders	71.4	65.5
Corporate and Other
Segment information
Total operating revenues	0.1	0.5
Other operation and maintenance	(1.8)	(1.6)
Depreciation and amortization	6.6	6.1
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	24.2	35.1
Income tax expense (benefit)	(32.9)	(30.7)
Net income (loss)	17.6	(18.3)
Net income attributed to common shareholders	$ 17.6	(18.3)
Bishop Hill III | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%
Coyote Ridge | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	80.00%
Upstream | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%
Blooming Grove | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%
Tatanka Ridge | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	85.00%
JayhawkWindLLC | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%
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	$ 0.9	0.2
ATC Holdco | Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%
Utility Operations
Segment information
Other operation and maintenance	$ 474.4	456.6
Depreciation and amortization	238.1	220.7
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	158.1	161.3
Income tax expense (benefit)	97.9	99.6
Net income (loss)	393.4	380.6
Net income attributed to common shareholders	393.1	380.3
Utility Operations | Wisconsin
Segment information
Other operation and maintenance	341.9	330.8
Depreciation and amortization	176.2	165.4
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	140.1	143.1
Income tax expense (benefit)	48.1	51.2
Net income (loss)	256.6	247.0
Net income attributed to common shareholders	256.3	246.7
Utility Operations | Illinois
Segment information
Other operation and maintenance	109.3	104.1
Depreciation and amortization	52.7	47.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	16.5	16.0
Income tax expense (benefit)	41.4	39.5
Net income (loss)	112.1	107.3
Net income attributed to common shareholders	112.1	107.3
Utility Operations | Other States
Segment information
Other operation and maintenance	23.2	21.7
Depreciation and amortization	9.2	7.8
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	1.5	2.2
Income tax expense (benefit)	8.4	8.9
Net income (loss)	24.7	26.3
Net income attributed to common shareholders	24.7	26.3
External Revenues
Segment information
Total operating revenues	2,691.4	2,108.6
External Revenues | Reconciling Eliminations
Segment information
Total operating revenues	0.0	0.0
External Revenues | Electric Transmission
Segment information
Total operating revenues	0.0	0.0
External Revenues | Non-Utility Energy Infrastructure
Segment information
Total operating revenues	22.9	15.2
External Revenues | Corporate and Other
Segment information
Total operating revenues	0.1	0.5
External Revenues | Utility Operations
Segment information
Total operating revenues	2,668.4	2,092.9
External Revenues | Utility Operations | Wisconsin
Segment information
Total operating revenues	1,731.7	1,498.9
External Revenues | Utility Operations | Illinois
Segment information
Total operating revenues	703.4	447.6
External Revenues | Utility Operations | Other States
Segment information
Total operating revenues	233.3	146.4
Intersegment Revenues
Segment information
Total operating revenues	0.0	0.0
Intersegment Revenues | Reconciling Eliminations
Segment information
Total operating revenues	(114.7)	(114.4)
Intersegment Revenues | Electric Transmission
Segment information
Total operating revenues	0.0	0.0
Intersegment Revenues | Non-Utility Energy Infrastructure
Segment information
Total operating revenues	114.7	114.4
Intersegment Revenues | Corporate and Other
Segment information
Total operating revenues	0.0	0.0
Intersegment Revenues | Utility Operations
Segment information
Total operating revenues	0.0	0.0
Intersegment Revenues | Utility Operations | Wisconsin
Segment information
Total operating revenues	0.0	0.0
Intersegment Revenues | Utility Operations | Illinois
Segment information
Total operating revenues	0.0	0.0
Intersegment Revenues | Utility Operations | Other States
Segment information
Total operating revenues	$ 0.0	$ 0.0