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, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2,010.8	$ 1,997.6	$ 5,381.7	$ 5,100.8
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	580.5	634.9	2,438.4	2,352.7
Total Utility Operations \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	580.0	634.1	2,437.8	2,351.2
Total Utility Operations \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	523.9	592.0	2,195.0	2,143.8
Total Utility Operations \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	376.2	433.5	1,546.2	1,530.6
Total Utility Operations \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	147.7	158.5	648.8	613.2
Total Utility Operations \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	82.6	90.4	224.4	234.5
Total Utility Operations \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(26.5)	(48.3)	18.4	(27.1)
Wisconsin \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	258.7	277.1	1,151.0	1,011.2
Wisconsin \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	233.1	238.2	1,089.0	977.4
Wisconsin \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	158.2	160.3	725.5	649.1
Wisconsin \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	74.9	77.9	363.5	328.3
Wisconsin \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	24.3	22.7	58.7	55.9
Wisconsin \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1.3	16.2	3.3	(22.1)
Illinois \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	243.4	280.5	955.4	1,039.8
Illinois \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	220.8	285.7	804.2	882.5
Illinois \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	171.9	227.2	624.1	692.8
Illinois \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.9	58.5	180.1	189.7
Illinois \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	50.4	60.1	144.2	157.5
Illinois \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(27.8)	(65.3)	7.0	(0.2)
Other States \| Natural gas \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	77.9	76.5	331.4	300.2
Other States \| Natural gas \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	70.0	68.1	301.8	283.9
Other States \| Natural gas \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	46.1	46.0	196.6	188.7
Other States \| Natural gas \| Transferred over time \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	23.9	22.1	105.2	95.2
Other States \| Natural gas \| Transferred over time \| Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.9	7.6	21.5	21.1
Other States \| Natural gas \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 0.0	$ 0.8	$ 8.1	$ (4.8)