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, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2,088.3	$ 1,854.3	$ 7,189.1	$ 6,202.2
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	413.3	380.9	2,766.0	2,279.2
Total Utility Operations \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	412.6	380.1	2,763.8	2,277.5
Total Utility Operations \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	313.5	288.8	2,457.3	1,953.6
Total Utility Operations \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	226.1	215.3	1,756.7	1,416.4
Total Utility Operations \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	87.4	73.5	700.6	537.2
Total Utility Operations \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	70.4	70.2	304.9	281.7
Total Utility Operations \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.7	21.1	1.6	42.2
Wisconsin \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	179.5	163.7	1,190.7	964.8
Wisconsin \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	125.3	107.6	1,102.7	875.1
Wisconsin \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	84.4	76.8	733.5	599.3
Wisconsin \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.9	30.8	369.2	275.8
Wisconsin \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.1	19.3	76.0	70.4
Wisconsin \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	34.1	36.8	12.0	19.3
Illinois \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	185.2	168.0	1,225.0	1,027.3
Illinois \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	148.6	146.0	1,031.1	831.6
Illinois \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	115.9	114.0	808.7	651.1
Illinois \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	32.7	32.0	222.4	180.5
Illinois \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	43.5	44.7	201.0	187.3
Illinois \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(6.9)	(22.7)	(7.1)	8.4
Other States \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	47.9	48.4	348.1	285.4
Other States \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	39.6	35.2	323.5	246.9
Other States \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	25.8	24.5	214.5	166.0
Other States \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	13.8	10.7	109.0	80.9
Other States \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.8	6.2	27.9	24.0
Other States \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1.5	$ 7.0	$ (3.3)	$ 14.5