OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Operating Revenues
Total operating revenues	$ 2,691.4	$ 2,108.6
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,669.0	2,086.9
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	22.4	21.7
Total regulated revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,642.7	2,067.3
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,095.0	1,034.6
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,547.7	1,032.7
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	26.3	19.6
Reconciling Eliminations
Disaggregation of Operating Revenues
Total operating revenues	(114.7)	(114.4)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(14.9)	(15.7)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(99.8)	(98.7)
Reconciling Eliminations | Total regulated revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(13.3)	(14.1)
Reconciling Eliminations | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(13.3)	(14.1)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(1.6)	(1.6)
Total Utility Operations
Disaggregation of Operating Revenues
Total operating revenues	2,668.4	2,092.9
Total Utility Operations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	22.3	21.6
Total Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,646.1	2,071.3
Total Utility Operations | Total regulated revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,641.4	2,066.9
Total Utility Operations | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,095.0	1,034.6
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,546.4	1,032.3
Total Utility Operations | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.7	4.4
Wisconsin
Disaggregation of Operating Revenues
Total operating revenues	1,731.7	1,498.9
Wisconsin | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	9.4	5.4
Wisconsin | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,722.3	1,493.5
Wisconsin | Total regulated revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,722.3	1,493.5
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,095.0	1,034.6
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	627.3	458.9
Wisconsin | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Illinois
Disaggregation of Operating Revenues
Total operating revenues	703.4	447.6
Illinois | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	9.9	14.0
Illinois | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	693.5	433.6
Illinois | Total regulated revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	693.5	433.6
Illinois | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	693.5	433.6
Illinois | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Other States
Disaggregation of Operating Revenues
Total operating revenues	233.3	146.4
Other States | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	3.0	2.2
Other States | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	230.3	144.2
Other States | Total regulated revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	225.6	139.8
Other States | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Other States | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	225.6	139.8
Other States | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.7	4.4
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Total operating revenues	137.6	129.6
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	37.8	30.9
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	99.8	98.7
Non-Utility Energy Infrastructure | Total regulated revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.6	14.5
Non-Utility Energy Infrastructure | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.6	14.5
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	23.2	16.4
Corporate and Other
Disaggregation of Operating Revenues
Total operating revenues	0.1	0.5
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.4
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.1	0.1
Corporate and Other | Total regulated revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Corporate and Other | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Corporate and Other | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0
Corporate and Other | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 0.0	$ 0.4