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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,638.6	$ 1,587.6	$ 5,268.5	$ 5,528.0
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	407.8	385.4	1,965.7	2,206.6
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,624.2	1,574.5	5,224.0	5,485.4
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	407.0	384.9	1,964.0	2,203.7
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	289.1	280.2	1,698.1	1,972.2
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	222.0	209.4	1,236.5	1,392.1
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	67.1	70.8	461.6	580.1
Total Utility Operations | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	61.0	64.3	238.6	253.8
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	56.9	40.4	27.3	(22.3)
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,363.4	1,336.5	4,064.3	4,212.4
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	150.5	151.1	817.2	943.3
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	102.9	99.3	740.1	864.9
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	74.6	69.3	513.5	579.4
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.3	30.0	226.6	285.5
Wisconsin | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	15.7	14.4	56.8	52.5
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.9	37.4	20.3	25.9
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	213.5	190.0	901.6	967.5
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	213.4	190.0	901.5	967.4
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	157.2	146.9	725.4	816.3
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	128.6	117.6	568.9	625.7
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.6	29.3	156.5	190.6
Illinois | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.0	44.3	159.0	178.3
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	16.2	(1.2)	17.1	(27.2)
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	47.3	48.0	258.1	305.5
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	43.1	43.8	245.3	293.0
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	29.0	34.0	232.6	291.0
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	18.8	22.5	154.1	187.0
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	10.2	11.5	78.5	104.0
Other States | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	5.3	5.6	22.8	23.0
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 8.8	$ 4.2	$ (10.1)	$ (21.0)