OPERATING REVENUES - DISAGGREGATION OF ELECTRIC 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
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,767.6	1,608.6	4,546.8	4,231.7
Wisconsin \| Electric \| Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,767.6	1,608.6	4,546.8	4,231.7
Wisconsin \| Electric \| Transferred over time \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,614.7	1,483.2	4,189.9	3,896.0
Wisconsin \| Electric \| Transferred over time \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	712.1	658.1	1,814.4	1,715.0
Wisconsin \| Electric \| Transferred over time \| Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	531.8	498.8	1,428.7	1,343.2
Wisconsin \| Electric \| Transferred over time \| Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	363.6	318.8	924.2	814.9
Wisconsin \| Electric \| Transferred over time \| Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.2	7.5	22.6	22.9
Wisconsin \| Electric \| Transferred over time \| Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	29.1	30.7	86.6	84.2
Wisconsin \| Electric \| Transferred over time \| Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	111.0	84.3	228.1	212.8
Wisconsin \| Electric \| Transferred over time \| Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	3.3	2.2	22.5	20.6
Wisconsin \| Electric \| Transferred over time \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 9.5	$ 8.2	$ 19.7	$ 18.1