OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Operating Revenues
Operating revenues	$ 1,590.2	$ 1,672.5	$ 3,967.6	$ 3,959.0
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,563.9	1,673.6	3,940.4	3,963.6
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	26.3	(1.1)	27.2	(4.6)
Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,546.7	1,660.6	3,904.9	3,939.0
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,021.9	1,084.2	2,083.7	2,151.9
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	524.8	576.4	1,821.2	1,787.1
Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	524.1	579.1	1,818.8	1,777.4
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	17.2	13.0	35.5	24.6
Total Utility Operations
Disaggregation of Operating Revenues
Operating revenues	1,565.0	1,665.9	3,920.1	3,932.2
Total Utility Operations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	14.8	(1.4)	9.2	(5.0)
Total Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,550.2	1,667.3	3,910.9	3,937.2
Total Utility Operations | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,546.0	1,663.3	3,902.5	3,929.3
Total Utility Operations | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,021.9	1,084.2	2,083.7	2,151.9
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	524.1	579.1	1,818.8	1,777.4
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	4.0	8.4	7.9
Wisconsin
Disaggregation of Operating Revenues
Operating revenues	1,253.3	1,325.5	2,886.7	2,914.6
Wisconsin | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	4.1	4.9	10.8	8.3
Wisconsin | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,249.2	1,320.6	2,875.9	2,906.3
Wisconsin | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,249.2	1,320.6	2,875.9	2,906.3
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,021.9	1,084.2	2,083.7	2,151.9
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	227.3	236.4	792.2	754.4
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	242.9	268.0	779.4	775.3
Illinois | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	10.1	(5.9)	1.9	(6.2)
Illinois | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	232.8	273.9	777.5	781.5
Illinois | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	232.8	273.8	777.4	781.4
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	232.8	273.8	777.4	781.4
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	0.1
Other States
Disaggregation of Operating Revenues
Operating revenues	68.8	72.4	254.0	242.3
Other States | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.6	(0.4)	(3.5)	(7.1)
Other States | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	68.2	72.8	257.5	249.4
Other States | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	64.0	68.9	249.2	241.6
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	64.0	68.9	249.2	241.6
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	3.9	8.3	7.8
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Operating revenues	123.3	117.0	251.1	235.1
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	25.1	19.3	54.8	41.3
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	98.2	97.7	196.3	193.8
Non-Utility Energy Infrastructure | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	9.8	10.0	26.2	24.9
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	9.8	10.0	26.2	24.9
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	15.3	9.3	28.6	16.4
Corporate and Other
Disaggregation of Operating Revenues
Operating revenues	0.9	3.1	2.6	4.5
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.8	2.8	2.3	4.1
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.1	0.3	0.3	0.4
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	0.8	2.8	2.3	4.1
Reconciling Eliminations
Disaggregation of Operating Revenues
Operating revenues	(99.0)	(113.5)	(206.2)	(212.8)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(12.2)	(15.8)	(27.6)	(19.0)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(86.8)	(97.7)	(178.6)	(193.8)
Reconciling Eliminations | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	(9.1)	(12.7)	(23.8)	(15.2)
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	(9.1)	(12.7)	(23.8)	(15.2)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (3.1)	$ (3.1)	$ (3.8)	$ (3.8)