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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,743.4	$ 1,810.0	$ 4,347.9	$ 4,668.2
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	534.6	577.3	1,898.3	2,184.6
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	534.0	576.8	1,897.4	2,183.9
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	428.0	472.9	1,664.8	2,065.7
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	317.1	353.7	1,201.1	1,441.9
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	110.9	119.2	463.7	623.8
Total Utility Operations | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	80.0	75.4	211.5	191.8
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	26.0	28.5	21.1	(73.6)
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	215.1	239.9	801.1	1,024.3
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	178.1	170.7	767.5	1,020.7
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	124.9	120.1	522.5	674.9
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	53.2	50.6	245.0	345.8
Wisconsin | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	21.3	20.4	51.1	49.3
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	15.7	48.8	(17.5)	(45.7)
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	255.5	260.0	859.3	837.7
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	203.6	222.4	685.6	709.2
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	162.1	180.0	537.1	548.9
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	41.5	42.4	148.5	160.3
Illinois | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	52.5	48.8	142.6	125.4
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(0.6)	(11.2)	31.1	3.1
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	63.4	76.9	237.0	321.9
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	46.3	79.8	211.7	335.8
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	30.1	53.6	141.5	218.1
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	16.2	26.2	70.2	117.7
Other States | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.2	6.2	17.8	17.1
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 10.9	$ (9.1)	$ 7.5	$ (31.0)