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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,991.1	$ 1,733.0	$ 7,011.9	$ 6,056.3
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	523.6	426.0	3,057.5	2,520.1
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,964.5	1,714.3	6,914.6	5,993.7
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	522.8	425.4	3,055.2	2,517.5
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	480.2	360.3	2,874.6	2,037.3
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	330.6	259.6	1,990.5	1,454.6
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	149.6	100.7	884.1	582.7
Total Utility Operations | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	67.3	65.5	266.1	248.3
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(24.7)	(0.4)	(85.5)	231.9
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,671.8	1,453.6	5,156.7	4,471.8
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	234.9	169.0	1,311.2	1,009.0
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	197.6	134.8	1,273.2	937.6
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	122.1	89.4	823.1	625.6
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	75.5	45.4	450.1	312.0
Wisconsin | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	15.9	16.1	59.4	58.3
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	21.4	18.1	(21.4)	13.1
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	224.0	208.3	1,346.1	1,167.9
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	224.0	208.3	1,346.1	1,167.9
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	229.6	187.2	1,200.2	874.4
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	177.0	146.5	911.1	679.5
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	52.6	40.7	289.1	194.9
Illinois | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	46.1	43.4	181.5	166.4
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(51.7)	(22.3)	(35.6)	127.1
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	68.7	52.4	411.8	354.0
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	63.9	48.1	397.9	340.6
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	53.0	38.3	401.2	225.3
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.5	23.7	256.3	149.5
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	21.5	14.6	144.9	75.8
Other States | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	5.3	6.0	25.2	23.6
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 5.6	$ 3.8	$ (28.5)	$ 91.7