OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - 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
Revenues from contracts with customers	$ 1,810.0	$ 2,121.2	$ 4,668.2	$ 5,020.8
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	577.3	868.5	2,184.6	2,533.9
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,760.0	2,093.1	4,576.1	4,950.1
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	576.8	867.5	2,183.9	2,532.4
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	472.9	747.5	2,065.7	2,394.4
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	353.7	530.6	1,441.9	1,659.9
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	119.2	216.9	623.8	734.5
Total Utility Operations | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	75.4	78.5	191.8	198.8
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.5	41.5	(73.6)	(60.8)
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,418.4	1,550.6	3,406.6	3,484.9
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	239.9	329.5	1,024.3	1,076.3
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	170.7	300.6	1,020.7	1,075.6
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	120.1	198.5	674.9	701.0
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	50.6	102.1	345.8	374.6
Wisconsin | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.4	18.0	49.3	43.5
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.8	10.9	(45.7)	(42.8)
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	260.0	442.2	837.7	1,122.1
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	260.0	442.2	837.7	1,122.1
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	222.4	346.8	709.2	970.6
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	180.0	268.6	548.9	734.1
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.4	78.2	160.3	236.5
Illinois | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.8	54.5	125.4	135.4
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(11.2)	40.9	3.1	16.1
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	81.6	100.3	331.8	343.1
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	76.9	95.8	321.9	334.0
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	79.8	100.1	335.8	348.2
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	53.6	63.5	218.1	224.8
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	26.2	36.6	117.7	123.4
Other States | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.2	6.0	17.1	19.9
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (9.1)	$ (10.3)	$ (31.0)	$ (34.1)