OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Operating Revenues
Total operating revenues	$ 2,009.5	$ 1,772.0	$ 5,159.0	$ 4,452.2
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,997.6	1,743.4	5,100.8	4,347.9
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	11.9	28.6	58.2	104.3
Utility operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,938.0	1,683.1	4,975.8	4,232.1
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,303.1	1,148.5	2,623.1	2,333.8
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	634.9	534.6	2,352.7	1,898.3
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	59.6	60.3	125.0	115.8
Reconciling Eliminations
Disaggregation of Operating Revenues
Total operating revenues	(120.0)	(119.6)	(240.1)	(239.7)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(14.5)	(14.7)	(29.5)	(30.5)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(105.5)	(104.9)	(210.6)	(209.2)
Reconciling Eliminations | Utility operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(10.6)	(10.8)	(24.0)	(25.0)
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	(10.6)	(10.8)	(24.0)	(25.0)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(3.9)	(3.9)	(5.5)	(5.5)
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	634.1	534.0	2,351.2	1,897.4
Total Utility Operations | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	1,940.1	1,716.0	5,015.4	4,345.4
Total Utility Operations | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(2.5)	28.6	30.2	104.3
Total Utility Operations | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,942.6	1,687.4	4,985.2	4,241.1
Total Utility Operations | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,937.2	1,682.5	4,974.3	4,231.2
Total Utility Operations | Operating Segments | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,303.1	1,148.5	2,623.1	2,333.8
Total Utility Operations | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	634.1	534.0	2,351.2	1,897.4
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	5.4	4.9	10.9	9.9
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,303.1	1,148.5	2,623.1	2,333.8
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	277.1	215.1	1,011.2	801.1
Wisconsin | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	1,587.2	1,368.2	3,647.1	3,147.0
Wisconsin | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	7.0	4.6	12.8	12.1
Wisconsin | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,580.2	1,363.6	3,634.3	3,134.9
Wisconsin | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,580.2	1,363.6	3,634.3	3,134.9
Wisconsin | Operating Segments | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,303.1	1,148.5	2,623.1	2,333.8
Wisconsin | Operating Segments | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	277.1	215.1	1,011.2	801.1
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	280.5	255.5	1,039.8	859.3
Illinois | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	270.6	276.8	1,058.9	942.8
Illinois | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(9.9)	21.3	19.1	83.5
Illinois | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	280.5	255.5	1,039.8	859.3
Illinois | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	280.5	255.5	1,039.8	859.3
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	280.5	255.5	1,039.8	859.3
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	76.5	63.4	300.2	237.0
Other States | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	82.3	71.0	309.4	255.6
Other States | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.4	2.7	(1.7)	8.7
Other States | Operating Segments | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	81.9	68.3	311.1	246.9
Other States | Operating Segments | Utility operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	76.5	63.4	300.2	237.0
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	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	76.5	63.4	300.2	237.0
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	5.4	4.9	10.9	9.9
Non-Utility Energy Infrastructure | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	189.4	175.6	383.7	346.5
Non-Utility Energy Infrastructure | Operating Segments | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	69.5	70.7	145.1	137.3
Non-Utility Energy Infrastructure | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	119.9	104.9	238.6	209.2
Non-Utility Energy Infrastructure | Operating Segments | Utility operations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	11.4	11.4	25.5	25.9
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	11.4	11.4	25.5	25.9
Non-Utility Energy Infrastructure | Operating Segments | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	58.1	59.3	119.6	111.4
Corporate and Other | Operating Segments
Disaggregation of Operating Revenues
Total operating revenues	0.0	0.0	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	0.0	0.0
Corporate and Other | Operating Segments | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.0	0.0	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	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