OPERATING REVENUES - DISAGGREGATION OF ELECTRIC 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
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,345.9	1,303.1	2,779.2	2,623.1
Wisconsin \| Electric \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,345.9	1,303.1	2,779.2	2,623.1
Wisconsin \| Electric \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,268.1	1,200.4	2,575.2	2,412.8
Wisconsin \| Electric \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	528.9	511.9	1,102.3	1,056.9
Wisconsin \| Electric \| Transferred over time \| Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	445.3	423.3	896.9	844.4
Wisconsin \| Electric \| Transferred over time \| Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	286.6	257.8	560.6	496.1
Wisconsin \| Electric \| Transferred over time \| Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.3	7.4	15.4	15.4
Wisconsin \| Electric \| Transferred over time \| Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	27.4	25.8	57.5	53.5
Wisconsin \| Electric \| Transferred over time \| Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.1	65.7	117.1	128.5
Wisconsin \| Electric \| Transferred over time \| Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	5.4	5.6	19.2	18.4
Wisconsin \| Electric \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 4.9	$ 5.6	$ 10.2	$ 9.9