OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Operating Revenues
Total operating revenues	$ 2,680.2	$ 2,888.1
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,604.5	2,858.2
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	75.7	29.9
Utility operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,549.0	2,811.1
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.3	1,203.8
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,363.7	1,607.3
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	55.5	47.1
Reconciling Eliminations
Disaggregation of Operating Revenues
Total operating revenues	(120.1)	(124.1)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(15.8)	(22.7)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(104.3)	(101.4)
Reconciling Eliminations | Utility operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(14.2)	(21.1)
Reconciling Eliminations | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(14.2)	(21.1)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(1.6)	(1.6)
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,363.4	1,638.0
Total Utility Operations | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	2,629.4	2,846.0
Total Utility Operations | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	75.7	29.9
Total Utility Operations | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,553.7	2,816.1
Total Utility Operations | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,548.7	2,810.9
Total Utility Operations | Operating Segments | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.3	1,203.8
Total Utility Operations | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,363.4	1,607.1
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	5.0	5.2
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.3	1,203.8
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	586.0	784.4
Wisconsin | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	1,778.8	1,996.3
Wisconsin | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	7.5	8.1
Wisconsin | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,771.3	1,988.2
Wisconsin | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,771.3	1,988.2
Wisconsin | Operating Segments | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.3	1,203.8
Wisconsin | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	586.0	784.4
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
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	603.8	608.6
Illinois | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	666.0	599.7
Illinois | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	62.2	22.0
Illinois | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	603.8	577.7
Illinois | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	603.8	577.7
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
Illinois | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	603.8	577.7
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
Other States | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	173.6	245.0
Other States | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	184.6	250.0
Other States | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	6.0	(0.2)
Other States | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	178.6	250.2
Other States | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	173.6	245.0
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
Other States | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	173.6	245.0
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	5.0	5.2
Non-Utility Energy Infrastructure | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	170.9	166.2
Non-Utility Energy Infrastructure | Operating Segments | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	66.6	64.8
Non-Utility Energy Infrastructure | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	104.3	101.4
Non-Utility Energy Infrastructure | Operating Segments | Utility operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.5	21.3
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
Non-Utility Energy Infrastructure | Operating Segments | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.5	21.3
Non-Utility Energy Infrastructure | Operating Segments | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	52.1	43.5
Corporate and Other | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	0.0	0.0
Corporate and Other | Operating Segments | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Corporate and Other | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.0	0.0
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
Corporate and Other | Operating Segments | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	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
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