SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment information
Number of reportable segments | segment			6
Operating revenues	$ 1,548.7	$ 1,590.2	$ 3,657.3	$ 3,967.6
Other operation and maintenance	473.1	503.6	928.8	1,054.2
Depreciation and amortization	242.5	229.9	481.6	456.3
Operating income (loss)	338.8	314.6	965.4	857.4
Equity in earnings of transmission affiliates	52.9	36.9	92.7	73.0
Interest expense	124.4	124.1	253.8	248.5
Wisconsin
Segment information
Operating revenues	1,206.2	1,253.3	2,705.1	2,886.7
Illinois
Segment information
Operating revenues	261.2	242.9	708.8	779.4
Other States
Segment information
Operating revenues	66.7	68.8	213.1	254.0
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
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliates	52.9	36.9	92.7	73.0
Interest expense	4.9	2.7	$ 9.7	5.3
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities			33.00%
Operating revenues	125.3	123.3	$ 254.9	251.1
Other operation and maintenance	7.6	6.8	12.8	10.6
Depreciation and amortization	24.4	22.9	48.9	45.5
Operating income (loss)	91.4	91.3	182.9	184.0
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	15.1	15.5	30.4	31.2
Corporate and Other
Segment information
Operating revenues	0.9	0.9	1.4	2.6
Other operation and maintenance	3.5	1.8	1.9	0.8
Depreciation and amortization	6.4	6.0	12.5	12.4
Operating income (loss)	(9.0)	(7.1)	(13.2)	(11.0)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	32.6	36.5	67.7	71.6
Reconciling Eliminations
Segment information
Other operation and maintenance	(1.0)	0.7	(5.5)	0.0
Depreciation and amortization	(12.8)	(3.6)	(25.0)	(8.2)
Operating income (loss)	(86.7)	(87.0)	(173.2)	(174.2)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	$ (86.8)	(89.1)	$ (173.9)	(178.6)
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%
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	(0.5)	$ 0.6	(0.9)
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	$ 463.0	494.3	$ 919.6	1,042.8
Depreciation and amortization	224.5	204.6	445.2	406.6
Operating income (loss)	343.1	317.4	968.9	858.6
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	158.6	158.5	319.9	319.0
Utility Operations | Wisconsin
Segment information
Other operation and maintenance	349.4	363.9	680.2	756.6
Depreciation and amortization	167.6	152.9	333.0	303.9
Operating income (loss)	277.6	270.2	704.4	632.0
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	140.0	142.7	283.1	286.1
Utility Operations | Illinois
Segment information
Other operation and maintenance	93.2	107.0	197.3	235.2
Depreciation and amortization	48.6	45.0	96.1	89.5
Operating income (loss)	59.5	42.6	221.1	180.5
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	16.1	13.9	32.1	28.7
Utility Operations | Other States
Segment information
Other operation and maintenance	20.4	23.4	42.1	51.0
Depreciation and amortization	8.3	6.7	16.1	13.2
Operating income (loss)	6.0	4.6	43.4	46.1
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	2.5	1.9	4.7	4.2
External Revenues
Segment information
Operating revenues	1,548.7	1,590.2	3,657.3	3,967.6
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	13.7	24.3	28.9	44.9
External Revenues | Corporate and Other
Segment information
Operating revenues	0.9	0.9	1.4	2.6
External Revenues | Reconciling Eliminations
Segment information
Operating revenues	0.0	0.0	0.0	0.0
External Revenues | Utility Operations
Segment information
Operating revenues	1,534.1	1,565.0	3,627.0	3,920.1
External Revenues | Utility Operations | Wisconsin
Segment information
Operating revenues	1,206.2	1,253.3	2,705.1	2,886.7
External Revenues | Utility Operations | Illinois
Segment information
Operating revenues	261.2	242.9	708.8	779.4
External Revenues | Utility Operations | Other States
Segment information
Operating revenues	66.7	68.8	213.1	254.0
Intersegment Revenues
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Revenues | Electric Transmission
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Revenues | Non-Utility Energy Infrastructure
Segment information
Operating revenues	111.6	99.0	226.0	206.2
Intersegment Revenues | Corporate and Other
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Revenues | Reconciling Eliminations
Segment information
Operating revenues	(111.6)	(99.0)	(226.0)	(206.2)
Intersegment Revenues | Utility Operations
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Revenues | Utility Operations | Wisconsin
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Revenues | Utility Operations | Illinois
Segment information
Operating revenues	0.0	0.0	0.0	0.0
Intersegment Revenues | Utility Operations | Other States
Segment information
Operating revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0