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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,587.6	$ 1,635.7	$ 5,528.0	$ 5,599.3
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	385.4	403.8	2,206.6	2,190.9
Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	384.9	406.5	2,203.7	2,183.9
Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	280.2	281.7	1,972.2	1,915.4
Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	209.4	204.7	1,392.1	1,346.9
Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	70.8	77.0	580.1	568.5
Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	64.3	62.1	253.8	248.5
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,574.5	1,628.9	5,485.4	5,566.1
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	384.9	406.5	2,203.7	2,183.9
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.4	62.7	(22.3)	20.0
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,336.5	1,385.7	4,212.4	4,292.0
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	151.1	167.4	943.3	921.8
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	99.3	106.9	864.9	843.1
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	69.3	71.9	579.4	556.7
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	30.0	35.0	285.5	286.4
Wisconsin | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.4	13.9	52.5	51.3
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	37.4	46.6	25.9	27.4
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	190.0	193.3	967.5	974.8
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	190.0	193.2	967.4	974.6
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	146.9	142.1	816.3	795.1
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	117.6	112.7	625.7	609.0
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	29.3	29.4	190.6	186.1
Illinois | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	44.3	43.3	178.3	175.6
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(1.2)	7.8	(27.2)	3.9
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.0	49.9	305.5	299.3
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	43.8	45.9	293.0	287.5
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	34.0	32.7	291.0	277.2
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	22.5	20.1	187.0	181.2
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	11.5	12.6	104.0	96.0
Other States | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	5.6	4.9	23.0	21.6
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 4.2	$ 8.3	$ (21.0)	$ (11.3)