SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)
Segment information
Number of reportable segments | segment					6
Total operating revenues	$ 1,676.2		$ 1,548.7		$ 4,367.6	$ 3,657.3
Other operation and maintenance	463.8		473.1		943.7	928.8
Depreciation and amortization	266.2		242.5		527.6	481.6
Equity in earnings of transmission affiliates	41.3		52.9		83.9	92.7
Interest expense	120.0		124.4		239.5	253.8
Income tax expense (benefit)	54.1		53.8		129.0	143.8
Net income (loss)	275.7		242.1		786.0	694.7
Net income (loss) attributed to common shareholders	276.0	$ 510.1	241.6	$ 452.5	786.1	694.1
Reconciling Eliminations
Segment information
Other operation and maintenance	(3.9)		(1.0)		(5.5)	(5.5)
Depreciation and amortization	(14.7)		(12.8)		(29.0)	(25.0)
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	(85.2)		(86.8)		(170.9)	(173.9)
Income tax expense (benefit)	0.0		0.0		0.0	0.0
Net income (loss)	0.0		0.0		0.0	0.0
Net income (loss) attributed to common shareholders	0.0		0.0		0.0	0.0
Wisconsin
Segment information
Total operating revenues	1,307.5		1,206.2		3,039.2	2,705.1
Illinois
Segment information
Total operating revenues	275.5		261.2		978.9	708.8
Other States
Segment information
Total operating revenues	72.1		66.7		305.4	213.1
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
Equity in earnings of transmission affiliates	41.3		52.9		83.9	92.7
Interest expense	4.8		4.9		9.7	9.7
Income tax expense (benefit)	9.4		14.5		19.2	24.4
Net income (loss)	27.0		33.5		55.0	58.5
Net income (loss) attributed to common shareholders	27.0		33.5		$ 55.0	58.5
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities					33.00%
Total operating revenues	133.5		125.3		$ 271.1	254.9
Other operation and maintenance	12.4		7.6		21.3	12.8
Depreciation and amortization	31.3		24.4		62.3	48.9
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	17.9		15.1		35.9	30.4
Income tax expense (benefit)	0.7		11.2		0.8	22.4
Net income (loss)	68.2		65.0		139.5	130.3
Net income (loss) attributed to common shareholders	68.8		64.8		140.2	130.3
Corporate and Other
Segment information
Total operating revenues	0.1		0.9		0.2	1.4
Other operation and maintenance	(2.8)		3.5		(4.6)	1.9
Depreciation and amortization	6.4		6.4		13.0	12.5
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	24.6		32.6		48.8	67.7
Income tax expense (benefit)	4.1		(6.9)		(28.8)	(37.6)
Net income (loss)	(12.4)		(21.4)		5.2	(39.7)
Net income (loss) attributed to common shareholders	$ (12.4)		(21.4)		$ 5.2	(39.7)
Bishop Hill III | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%				90.00%
Coyote Ridge | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	80.00%				80.00%
Upstream | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%				90.00%
Blooming Grove | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%				90.00%
Tatanka Ridge | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	85.00%				85.00%
JayhawkWindLLC | Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest	90.00%				90.00%
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	$ 0.6		0.4		$ 1.5	0.6
ATC Holdco | Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%				75.00%
Utility Operations
Segment information
Other operation and maintenance	$ 458.1		463.0		$ 932.5	919.6
Depreciation and amortization	243.2		224.5		481.3	445.2
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	157.9		158.6		316.0	319.9
Income tax expense (benefit)	39.9		35.0		137.8	134.6
Net income (loss)	192.9		165.0		586.3	545.6
Net income (loss) attributed to common shareholders	192.6		164.7		585.7	545.0
Utility Operations | Wisconsin
Segment information
Other operation and maintenance	346.1		349.4		688.0	680.2
Depreciation and amortization	179.8		167.6		356.0	333.0
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	139.8		140.0		279.9	283.1
Income tax expense (benefit)	23.1		21.8		71.2	73.0
Net income (loss)	146.8		130.1		403.4	377.1
Net income (loss) attributed to common shareholders	146.5		129.8		402.8	376.5
Utility Operations | Illinois
Segment information
Other operation and maintenance	90.8		93.2		200.1	197.3
Depreciation and amortization	54.0		48.6		106.7	96.1
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	16.6		16.1		33.1	32.1
Income tax expense (benefit)	16.0		12.2		57.4	51.7
Net income (loss)	43.6		32.1		155.7	139.4
Net income (loss) attributed to common shareholders	43.6		32.1		155.7	139.4
Utility Operations | Other States
Segment information
Other operation and maintenance	21.2		20.4		44.4	42.1
Depreciation and amortization	9.4		8.3		18.6	16.1
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	1.5		2.5		3.0	4.7
Income tax expense (benefit)	0.8		1.0		9.2	9.9
Net income (loss)	2.5		2.8		27.2	29.1
Net income (loss) attributed to common shareholders	2.5		2.8		27.2	29.1
External Revenues
Segment information
Total operating revenues	1,676.2		1,548.7		4,367.6	3,657.3
External Revenues | Reconciling Eliminations
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
External Revenues | Electric Transmission
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
External Revenues | Non-Utility Energy Infrastructure
Segment information
Total operating revenues	21.0		13.7		43.9	28.9
External Revenues | Corporate and Other
Segment information
Total operating revenues	0.1		0.9		0.2	1.4
External Revenues | Utility Operations
Segment information
Total operating revenues	1,655.1		1,534.1		4,323.5	3,627.0
External Revenues | Utility Operations | Wisconsin
Segment information
Total operating revenues	1,307.5		1,206.2		3,039.2	2,705.1
External Revenues | Utility Operations | Illinois
Segment information
Total operating revenues	275.5		261.2		978.9	708.8
External Revenues | Utility Operations | Other States
Segment information
Total operating revenues	72.1		66.7		305.4	213.1
Intersegment Revenues
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
Intersegment Revenues | Reconciling Eliminations
Segment information
Total operating revenues	(112.5)		(111.6)		(227.2)	(226.0)
Intersegment Revenues | Electric Transmission
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
Intersegment Revenues | Non-Utility Energy Infrastructure
Segment information
Total operating revenues	112.5		111.6		227.2	226.0
Intersegment Revenues | Corporate and Other
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
Intersegment Revenues | Utility Operations
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
Intersegment Revenues | Utility Operations | Wisconsin
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
Intersegment Revenues | Utility Operations | Illinois
Segment information
Total operating revenues	0.0		0.0		0.0	0.0
Intersegment Revenues | Utility Operations | Other States
Segment information
Total operating revenues	$ 0.0		$ 0.0		$ 0.0	$ 0.0