SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)
Segment information
Number of reportable segments | segment			6
Operating revenues	$ 1,651.0	$ 1,608.0	$ 5,308.3	$ 5,575.6
Other operation and maintenance	498.7	529.2	1,427.5	1,583.4
Depreciation and amortization	245.0	233.8	726.6	690.1
Operating income (loss)	370.2	310.9	1,335.6	1,168.3
Equity in earnings of transmission affiliates	40.1	38.7	132.8	111.7
Interest expense	122.0	125.8	375.8	374.3
Wisconsin
Segment information
Operating revenues	1,366.3	1,339.3	4,071.4	4,226.0
Illinois
Segment information
Operating revenues	221.9	198.0	930.7	977.4
Other States
Segment information
Operating revenues	48.3	48.9	261.4	302.9
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	40.1	38.7	132.8	111.7
Interest expense	4.9	3.0	$ 14.6	8.3
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities			33.00%
Operating revenues	123.5	119.4	$ 378.4	370.5
Other operation and maintenance	5.9	3.9	18.7	14.5
Depreciation and amortization	24.5	23.3	73.4	68.8
Operating income (loss)	91.3	90.3	274.2	274.3
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	15.0	15.5	45.4	46.7
Corporate and Other
Segment information
Operating revenues	0.6	1.3	2.0	3.9
Other operation and maintenance	1.0	1.3	2.9	2.1
Depreciation and amortization	6.5	6.0	19.0	18.4
Operating income (loss)	(6.9)	(6.0)	(20.1)	(17.0)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	30.4	35.9	98.1	107.5
Reconciling Eliminations
Segment information
Other operation and maintenance	(1.6)	0.0	(7.1)	0.0
Depreciation and amortization	(14.1)	(3.6)	(39.1)	(11.8)
Operating income (loss)	(85.7)	(86.8)	(258.9)	(261.0)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	$ (85.7)	(88.5)	$ (259.6)	(267.1)
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	$ 0.5	0.4	$ 1.1	(0.5)
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	$ 493.4	524.0	$ 1,413.0	1,566.8
Depreciation and amortization	228.1	208.1	673.3	614.7
Operating income (loss)	371.5	313.4	1,340.4	1,172.0
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	157.4	159.9	477.3	478.9
Utility Operations | Wisconsin
Segment information
Other operation and maintenance	363.8	400.2	1,044.0	1,156.8
Depreciation and amortization	169.7	155.6	502.7	459.5
Operating income (loss)	349.0	290.8	1,053.4	922.8
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	139.1	142.9	422.2	429.0
Utility Operations | Illinois
Segment information
Other operation and maintenance	109.2	101.8	306.5	337.0
Depreciation and amortization	49.9	45.7	146.0	135.2
Operating income (loss)	24.5	24.8	245.6	205.3
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	15.6	14.7	47.7	43.4
Utility Operations | Other States
Segment information
Other operation and maintenance	20.4	22.0	62.5	73.0
Depreciation and amortization	8.5	6.8	24.6	20.0
Operating income (loss)	(2.0)	(2.2)	41.4	43.9
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	2.7	2.3	7.4	6.5
External Revenues
Segment information
Operating revenues	1,651.0	1,608.0	5,308.3	5,575.6
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	13.9	20.5	42.8	65.4
External Revenues | Corporate and Other
Segment information
Operating revenues	0.6	1.3	2.0	3.9
External Revenues | Reconciling Eliminations
Segment information
Operating revenues	0.0	0.0	0.0	0.0
External Revenues | Utility Operations
Segment information
Operating revenues	1,636.5	1,586.2	5,263.5	5,506.3
External Revenues | Utility Operations | Wisconsin
Segment information
Operating revenues	1,366.3	1,339.3	4,071.4	4,226.0
External Revenues | Utility Operations | Illinois
Segment information
Operating revenues	221.9	198.0	930.7	977.4
External Revenues | Utility Operations | Other States
Segment information
Operating revenues	48.3	48.9	261.4	302.9
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	109.6	98.9	335.6	305.1
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	(109.6)	(98.9)	(335.6)	(305.1)
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