OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Operating Revenues
Total operating revenues	$ 1,830.0	$ 2,127.9	$ 4,718.1	$ 5,036.0
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,810.0	2,121.2	4,668.2	5,020.8
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	20.0	6.7	49.9	15.2
Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,755.8	2,089.6	4,566.9	4,942.5
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,178.5	1,221.1	2,382.3	2,408.6
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	577.3	868.5	2,184.6	2,533.9
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	54.2	31.6	101.3	78.3
Reconciling Eliminations
Disaggregation of Operating Revenues
Total operating revenues	(119.0)	(115.5)	(243.1)	(232.4)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(17.4)	(15.0)	(40.1)	(31.4)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(101.6)	(100.5)	(203.0)	(201.0)
Reconciling Eliminations | Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(13.6)	(11.0)	(34.7)	(25.8)
Reconciling Eliminations | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(13.6)	(11.0)	(34.7)	(25.8)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(3.8)	(4.0)	(5.4)	(5.6)
Total Utility Operations
Disaggregation of Operating Revenues
Total operating revenues	1,779.9	2,099.7	4,625.9	4,965.0
Total Utility Operations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	19.9	6.6	49.8	14.9
Total Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,760.0	2,093.1	4,576.1	4,950.1
Total Utility Operations | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,755.3	2,088.6	4,566.2	4,941.0
Total Utility Operations | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,178.5	1,221.1	2,382.3	2,408.6
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	576.8	867.5	2,183.9	2,532.4
Total Utility Operations | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.7	4.5	9.9	9.1
Wisconsin
Disaggregation of Operating Revenues
Total operating revenues	1,424.5	1,557.4	3,420.8	3,499.7
Wisconsin | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	6.1	6.8	14.2	14.8
Wisconsin | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,418.4	1,550.6	3,406.6	3,484.9
Wisconsin | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,418.4	1,550.6	3,406.6	3,484.9
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,178.5	1,221.1	2,382.3	2,408.6
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	239.9	329.5	1,024.3	1,076.3
Wisconsin | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Illinois
Disaggregation of Operating Revenues
Total operating revenues	273.5	442.4	873.2	1,124.5
Illinois | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	13.5	0.2	35.5	2.4
Illinois | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	260.0	442.2	837.7	1,122.1
Illinois | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	260.0	442.2	837.7	1,122.1
Illinois | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	260.0	442.2	837.7	1,122.1
Illinois | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other States
Disaggregation of Operating Revenues
Total operating revenues	81.9	99.9	331.9	340.8
Other States | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.3	(0.4)	0.1	(2.3)
Other States | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	81.6	100.3	331.8	343.1
Other States | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	76.9	95.8	321.9	334.0
Other States | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Other States | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	76.9	95.8	321.9	334.0
Other States | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.7	4.5	9.9	9.1
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Total operating revenues	169.0	143.6	335.2	303.1
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	67.4	43.1	132.2	102.1
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	101.6	100.5	203.0	201.0
Non-Utility Energy Infrastructure | Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.1	12.0	35.4	27.3
Non-Utility Energy Infrastructure | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.1	12.0	35.4	27.3
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	53.3	31.1	96.8	74.8
Corporate and Other
Disaggregation of Operating Revenues
Total operating revenues	0.1	0.1	0.1	0.3
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.1	0.1	0.1	0.3
Corporate and Other | Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Corporate and Other | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Corporate and Other | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Corporate and Other | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 0.0	$ 0.0	$ 0.0	$ 0.0