OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - 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
Operating revenues	$ 1,608.0	$ 1,643.7	$ 5,575.6	$ 5,602.7
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,587.6	1,635.7	5,528.0	5,599.3
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	20.4	8.0	47.6	3.4
Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,570.8	1,622.1	5,475.7	5,561.1
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.4	1,218.3	3,269.1	3,370.2
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	385.4	403.8	2,206.6	2,190.9
Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	384.9	406.5	2,203.7	2,183.9
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	16.8	13.6	52.3	38.2
Total Utility Operations
Disaggregation of Operating Revenues
Operating revenues	1,586.2	1,636.8	5,506.3	5,569.0
Total Utility Operations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	11.7	7.9	20.9	2.9
Total Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,574.5	1,628.9	5,485.4	5,566.1
Total Utility Operations | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,570.3	1,624.8	5,472.8	5,554.1
Total Utility Operations | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.4	1,218.3	3,269.1	3,370.2
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	384.9	406.5	2,203.7	2,183.9
Total Utility Operations | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.2	4.1	12.6	12.0
Wisconsin
Disaggregation of Operating Revenues
Operating revenues	1,339.3	1,388.7	4,226.0	4,303.3
Wisconsin | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	2.8	3.0	13.6	11.3
Wisconsin | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,336.5	1,385.7	4,212.4	4,292.0
Wisconsin | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,336.5	1,385.7	4,212.4	4,292.0
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.4	1,218.3	3,269.1	3,370.2
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	151.1	167.4	943.3	921.8
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	0.0	0.0
Illinois
Disaggregation of Operating Revenues
Operating revenues	198.0	197.9	977.4	973.2
Illinois | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	8.0	4.6	9.9	(1.6)
Illinois | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	190.0	193.3	967.5	974.8
Illinois | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	190.0	193.2	967.4	974.6
Illinois | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	190.0	193.2	967.4	974.6
Illinois | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.1	0.1	0.2
Other States
Disaggregation of Operating Revenues
Operating revenues	48.9	50.2	302.9	292.5
Other States | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.9	0.3	(2.6)	(6.8)
Other States | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.0	49.9	305.5	299.3
Other States | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	43.8	45.9	293.0	287.5
Other States | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	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	43.8	45.9	293.0	287.5
Other States | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.2	4.0	12.5	11.8
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Operating revenues	119.4	115.2	370.5	350.3
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.9	17.9	75.7	59.2
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	98.5	97.3	294.8	291.1
Non-Utility Energy Infrastructure | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	8.9	10.0	35.1	34.9
Non-Utility Energy Infrastructure | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	8.9	10.0	35.1	34.9
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	12.0	7.9	40.6	24.3
Corporate and Other
Disaggregation of Operating Revenues
Operating revenues	1.3	2.4	3.9	6.9
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1.3	2.3	3.6	6.4
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.0	0.1	0.3	0.5
Corporate and Other | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	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	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	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	1.3	2.3	3.6	6.4
Reconciling Eliminations
Disaggregation of Operating Revenues
Operating revenues	(98.9)	(110.7)	(305.1)	(323.5)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(9.1)	(13.4)	(36.7)	(32.4)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(89.8)	(97.3)	(268.4)	(291.1)
Reconciling Eliminations | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(8.4)	(12.7)	(32.2)	(27.9)
Reconciling Eliminations | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.0	0.0	0.0	0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(8.4)	(12.7)	(32.2)	(27.9)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (0.7)	$ (0.7)	$ (4.5)	$ (4.5)