SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment information
Number of reportable segments | segment							6
Total operating revenues	$ 1,746.5			$ 1,651.0			$ 6,114.1	$ 5,308.3
Other operation and maintenance	473.7			498.7			1,417.4	1,427.5
Depreciation and amortization	271.6			245.0			799.2	726.6
Equity in earnings of transmission affiliates	42.3			40.1			126.2	132.8
Interest expense	118.0			122.0			357.5	375.8
Income tax expense (benefit)	50.8			46.9			179.8	190.7
Net income (loss)	288.7			267.1			1,074.7	961.8
Net income (loss) attributed to common shareholders	290.0	$ 276.0	$ 510.1	266.8	$ 241.6	$ 452.5	1,076.1	960.9
External Revenues
Segment information
Total operating revenues	1,746.5			1,651.0			6,114.1	5,308.3
Intersegment Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
Utility Operations
Segment information
Other operation and maintenance	469.9			493.4			1,402.4	1,413.0
Depreciation and amortization	249.4			228.1			730.7	673.3
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	156.0			157.4			472.0	477.3
Income tax expense (benefit)	39.6			32.3			177.4	166.9
Net income (loss)	214.2			197.6			800.5	743.2
Net income (loss) attributed to common shareholders	213.9			197.3			799.6	742.3
Utility Operations | External Revenues
Segment information
Total operating revenues	1,727.6			1,636.5			6,051.1	5,263.5
Utility Operations | Intersegment Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
Reconciling Eliminations
Segment information
Other operation and maintenance	(1.6)			(1.6)			(7.1)	(7.1)
Depreciation and amortization	(15.3)			(14.1)			(44.3)	(39.1)
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	(85.1)			(85.7)			(256.0)	(259.6)
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
Reconciling Eliminations | External Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
Reconciling Eliminations | Intersegment Revenues
Segment information
Total operating revenues	$ (110.8)			(109.6)			$ (338.0)	(335.6)
ATC Holdco
Segment information
Equity method investment, ownership interest (as a percent)	75.00%						75.00%
Equity in earnings of transmission affiliates	$ 0.6			0.5			$ 2.1	1.1
Wisconsin
Segment information
Total operating revenues	1,458.6			1,366.3			4,497.8	4,071.4
Wisconsin | Utility Operations
Segment information
Other operation and maintenance	359.1			363.8			1,047.1	1,044.0
Depreciation and amortization	184.4			169.7			540.4	502.7
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	137.9			139.1			417.8	422.2
Income tax expense (benefit)	33.6			37.5			104.8	110.5
Net income (loss)	197.8			187.9			601.2	565.0
Net income (loss) attributed to common shareholders	197.5			187.6			600.3	564.1
Wisconsin | Utility Operations | External Revenues
Segment information
Total operating revenues	1,458.6			1,366.3			4,497.8	4,071.4
Wisconsin | Utility Operations | Intersegment Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
Illinois
Segment information
Total operating revenues	216.2			221.9			1,195.1	930.7
Illinois | Utility Operations
Segment information
Other operation and maintenance	91.2			109.2			291.3	306.5
Depreciation and amortization	55.4			49.9			162.1	146.0
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	16.5			15.6			49.6	47.7
Income tax expense (benefit)	7.0			(4.0)			64.4	47.7
Net income (loss)	19.1			13.0			174.8	152.4
Net income (loss) attributed to common shareholders	19.1			13.0			174.8	152.4
Illinois | Utility Operations | External Revenues
Segment information
Total operating revenues	216.2			221.9			1,195.1	930.7
Illinois | Utility Operations | Intersegment Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
Other States
Segment information
Total operating revenues	52.8			48.3			358.2	261.4
Other States | Utility Operations
Segment information
Other operation and maintenance	19.6			20.4			64.0	62.5
Depreciation and amortization	9.6			8.5			28.2	24.6
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	1.6			2.7			4.6	7.4
Income tax expense (benefit)	(1.0)			(1.2)			8.2	8.7
Net income (loss)	(2.7)			(3.3)			24.5	25.8
Net income (loss) attributed to common shareholders	(2.7)			(3.3)			24.5	25.8
Other States | Utility Operations | External Revenues
Segment information
Total operating revenues	52.8			48.3			358.2	261.4
Other States | Utility Operations | Intersegment Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
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	42.3			40.1			126.2	132.8
Interest expense	4.8			4.9			14.5	14.6
Income tax expense (benefit)	9.7			9.0			28.9	33.4
Net income (loss)	27.8			26.3			82.8	84.8
Net income (loss) attributed to common shareholders	27.8			26.3			82.8	84.8
Electric Transmission | External Revenues
Segment information
Total operating revenues	0.0			0.0			0.0	0.0
Electric Transmission | Intersegment Revenues
Segment information
Total operating revenues	$ 0.0			0.0			$ 0.0	0.0
Electric Transmission | ATC
Segment information
Equity method investment, ownership interest (as a percent)	60.00%						60.00%
Electric Transmission | ATC Holdco
Segment information
Equity method investment, ownership interest (as a percent)	75.00%						75.00%
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities							33.00%
Total operating revenues	$ 129.5			123.5			$ 400.6	378.4
Other operation and maintenance	9.9			5.9			31.2	18.7
Depreciation and amortization	31.2			24.5			93.5	73.4
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	17.6			15.0			53.5	45.4
Income tax expense (benefit)	5.0			13.4			5.8	35.8
Net income (loss)	62.8			62.9			202.3	193.2
Net income (loss) attributed to common shareholders	64.4			62.9			204.6	193.2
Non-Utility Energy Infrastructure | External Revenues
Segment information
Total operating revenues	18.7			13.9			62.6	42.8
Non-Utility Energy Infrastructure | Intersegment Revenues
Segment information
Total operating revenues	$ 110.8			109.6			$ 338.0	335.6
Non-Utility Energy Infrastructure | Bishop Hill III
Segment information
WEC's ownership interest	90.00%						90.00%
Non-Utility Energy Infrastructure | Coyote Ridge
Segment information
WEC's ownership interest	80.00%						80.00%
Non-Utility Energy Infrastructure | Upstream
Segment information
WEC's ownership interest	90.00%						90.00%
Non-Utility Energy Infrastructure | Blooming Grove
Segment information
WEC's ownership interest	90.00%						90.00%
Non-Utility Energy Infrastructure | Tatanka Ridge
Segment information
WEC's ownership interest	85.00%						85.00%
Non-Utility Energy Infrastructure | JayhawkWindLLC
Segment information
WEC's ownership interest	90.00%						90.00%
Corporate and Other
Segment information
Total operating revenues	$ 0.2			0.6			$ 0.4	2.0
Other operation and maintenance	(4.5)			1.0			(9.1)	2.9
Depreciation and amortization	6.3			6.5			19.3	19.0
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	24.7			30.4			73.5	98.1
Income tax expense (benefit)	(3.5)			(7.8)			(32.3)	(45.4)
Net income (loss)	(16.1)			(19.7)			(10.9)	(59.4)
Net income (loss) attributed to common shareholders	(16.1)			(19.7)			(10.9)	(59.4)
Corporate and Other | External Revenues
Segment information
Total operating revenues	0.2			0.6			0.4	2.0
Corporate and Other | Intersegment Revenues
Segment information
Total operating revenues	$ 0.0			$ 0.0			$ 0.0	$ 0.0