OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Operating Revenues
Total operating revenues	$ 1,957.4	$ 2,003.0	$ 6,675.5	$ 7,039.0
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,944.0	1,991.1	6,612.2	7,011.9
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	13.4	11.9	63.3	27.1
Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,895.2	1,960.5	6,462.1	6,903.0
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,457.8	1,436.9	3,840.1	3,845.5
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	437.4	523.6	2,622.0	3,057.5
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.8	30.6	150.1	108.9
Reconciling Eliminations
Disaggregation of Operating Revenues
Total operating revenues	(115.3)	(113.3)	(358.4)	(345.7)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(14.0)	(12.8)	(54.1)	(44.2)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(101.3)	(100.5)	(304.3)	(301.5)
Reconciling Eliminations | Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(12.3)	(11.3)	(47.0)	(37.1)
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	(12.3)	(11.3)	(47.0)	(37.1)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(1.7)	(1.5)	(7.1)	(7.1)
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	436.8	522.8	2,620.7	3,055.2
Total Utility Operations | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	1,912.9	1,976.3	6,538.8	6,941.3
Total Utility Operations | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	13.4	11.8	63.2	26.7
Total Utility Operations | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,899.5	1,964.5	6,475.6	6,914.6
Total Utility Operations | Operating Segments | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,894.6	1,959.7	6,460.8	6,900.7
Total Utility Operations | Operating Segments | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,457.8	1,436.9	3,840.1	3,845.5
Total Utility Operations | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	436.8	522.8	2,620.7	3,055.2
Total Utility Operations | Operating Segments | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.9	4.8	14.8	13.9
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,457.8	1,436.9	3,840.1	3,845.5
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	159.5	234.9	1,183.8	1,311.2
Wisconsin | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	1,622.0	1,676.8	5,042.8	5,176.5
Wisconsin | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	4.7	5.0	18.9	19.8
Wisconsin | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,617.3	1,671.8	5,023.9	5,156.7
Wisconsin | Operating Segments | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,617.3	1,671.8	5,023.9	5,156.7
Wisconsin | Operating Segments | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,457.8	1,436.9	3,840.1	3,845.5
Wisconsin | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	159.5	234.9	1,183.8	1,311.2
Wisconsin | Operating Segments | 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 | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	234.8	224.0	1,072.5	1,346.1
Illinois | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	243.3	230.3	1,116.5	1,354.8
Illinois | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	8.5	6.3	44.0	8.7
Illinois | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	234.8	224.0	1,072.5	1,346.1
Illinois | Operating Segments | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	234.8	224.0	1,072.5	1,346.1
Illinois | Operating Segments | 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 | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	234.8	224.0	1,072.5	1,346.1
Illinois | Operating Segments | 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 | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.5	63.9	364.4	397.9
Other States | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	47.6	69.2	379.5	410.0
Other States | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.2	0.5	0.3	(1.8)
Other States | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	47.4	68.7	379.2	411.8
Other States | Operating Segments | Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.5	63.9	364.4	397.9
Other States | Operating Segments | 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 | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.5	63.9	364.4	397.9
Other States | Operating Segments | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.9	4.8	14.8	13.9
Non-Utility Energy Infrastructure | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	159.8	139.9	495.0	443.0
Non-Utility Energy Infrastructure | Operating Segments | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	58.5	39.4	190.7	141.5
Non-Utility Energy Infrastructure | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	101.3	100.5	304.3	301.5
Non-Utility Energy Infrastructure | Operating Segments | Utility Operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	12.9	12.1	48.3	39.4
Non-Utility Energy Infrastructure | Operating Segments | 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 | Operating Segments | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	12.9	12.1	48.3	39.4
Non-Utility Energy Infrastructure | Operating Segments | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	45.6	27.3	142.4	102.1
Corporate and Other | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	0.0	0.1	0.1	0.4
Corporate and Other | Operating Segments | 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 | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.0	0.1	0.1	0.4
Corporate and Other | Operating Segments | 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 | Operating Segments | 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 | Operating Segments | 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 | Operating Segments | 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