OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2,604.5	$ 2,858.2
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,363.7	1,607.3
Total Utility Operations \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,363.4	1,638.0
Total Utility Operations \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,236.8	1,592.8
Total Utility Operations \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	884.0	1,088.2
Total Utility Operations \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	352.8	504.6
Total Utility Operations \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	131.5	129.9
Total Utility Operations \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(4.9)	(84.7)
Wisconsin \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	586.0	784.4
Wisconsin \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	589.4	850.0
Wisconsin \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	397.6	554.8
Wisconsin \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	191.8	295.2
Wisconsin \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	29.8	28.9
Wisconsin \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(33.2)	(94.5)
Illinois \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	603.8	608.6
Illinois \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	482.0	486.8
Illinois \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	375.0	368.9
Illinois \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	107.0	117.9
Illinois \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	90.1	90.1
Illinois \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.7	31.7
Other States \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	173.6	245.0
Other States \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	165.4	256.0
Other States \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	111.4	164.5
Other States \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	54.0	91.5
Other States \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	11.6	10.9
Other States \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (3.4)	$ (21.9)