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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,733.0	$ 1,638.6	$ 6,056.3	$ 5,268.5
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	426.0	407.8	2,520.1	1,965.7
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,714.3	1,624.2	5,993.7	5,224.0
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	425.4	407.0	2,517.5	1,964.0
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	360.3	289.1	2,037.3	1,698.1
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	259.6	222.0	1,454.6	1,236.5
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	100.7	67.1	582.7	461.6
Total Utility Operations | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	65.5	61.0	248.3	238.6
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(0.4)	56.9	231.9	27.3
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,453.6	1,363.4	4,471.8	4,064.3
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	169.0	150.5	1,009.0	817.2
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	134.8	102.9	937.6	740.1
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	89.4	74.6	625.6	513.5
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	45.4	28.3	312.0	226.6
Wisconsin | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	16.1	15.7	58.3	56.8
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	18.1	31.9	13.1	20.3
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	208.3	213.5	1,167.9	901.6
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	208.3	213.4	1,167.9	901.5
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	187.2	157.2	874.4	725.4
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	146.5	128.6	679.5	568.9
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.7	28.6	194.9	156.5
Illinois | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	43.4	40.0	166.4	159.0
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(22.3)	16.2	127.1	17.1
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	52.4	47.3	354.0	258.1
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.1	43.1	340.6	245.3
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	38.3	29.0	225.3	232.6
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	23.7	18.8	149.5	154.1
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.6	10.2	75.8	78.5
Other States | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.0	5.3	23.6	22.8
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 3.8	$ 8.8	$ 91.7	$ (10.1)