OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2,228.3	$ 2,088.3	$ 7,610.0	$ 7,189.1
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	388.1	413.3	2,826.5	2,766.0
Total Utility Operations \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	387.8	412.6	2,825.6	2,672.1
Total Utility Operations \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	307.0	313.5	2,502.0	2,363.2
Total Utility Operations \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	218.0	226.1	1,764.2	1,673.4
Total Utility Operations \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	89.0	87.4	737.8	689.8
Total Utility Operations \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	69.9	70.4	294.3	290.1
Total Utility Operations \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	10.9	28.7	29.3	18.8
Wisconsin \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	155.0	179.5	1,306.0	1,190.7
Wisconsin \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	133.6	125.3	1,222.6	1,102.7
Wisconsin \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	89.7	84.4	815.2	733.5
Wisconsin \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	43.9	40.9	407.4	369.2
Wisconsin \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.9	20.1	79.6	76.0
Wisconsin \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.5	34.1	3.8	12.0
Illinois \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	177.9	185.2	1,133.3	1,133.3
Illinois \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	132.8	148.6	937.0	937.0
Illinois \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	101.3	115.9	725.4	725.4
Illinois \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.5	32.7	211.6	211.6
Illinois \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.0	43.5	186.2	186.2
Illinois \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	3.1	(6.9)	10.1	10.1
Other States \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	54.9	47.9	386.3	348.1
Other States \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.6	39.6	342.4	323.5
Other States \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	27.0	25.8	223.6	214.5
Other States \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	13.6	13.8	118.8	109.0
Other States \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.0	6.8	28.5	27.9
Other States \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 7.3	$ 1.5	$ 15.4	$ (3.3)