OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Operating Revenues
Revenues	$ 1,672.5	$ 1,631.5	$ 3,959.0	$ 3,936.0
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,673.6		3,963.6
Other operating revenues
Disaggregation of Operating Revenues
Revenues	(1.1)		(4.6)
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,084.2		2,151.9
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	576.4		1,787.1
Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,660.6		3,939.0
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	13.0		24.6
Public Utility
Disaggregation of Operating Revenues
Revenues	1,665.9		3,932.2
Public Utility | Other operating revenues
Disaggregation of Operating Revenues
Revenues	(1.4)		(5.0)
Public Utility | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,667.3		3,937.2
Public Utility | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,084.2		2,151.9
Public Utility | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	579.1		1,777.4
Public Utility | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,663.3		3,929.3
Public Utility | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	4.0		7.9
Wisconsin | Public Utility
Disaggregation of Operating Revenues
Revenues	1,325.5		2,914.6
Wisconsin | Public Utility | Other operating revenues
Disaggregation of Operating Revenues
Revenues	4.9		8.3
Wisconsin | Public Utility | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,320.6		2,906.3
Wisconsin | Public Utility | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,084.2		2,151.9
Wisconsin | Public Utility | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	236.4		754.4
Wisconsin | Public Utility | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,320.6		2,906.3
Wisconsin | Public Utility | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Illinois | Public Utility
Disaggregation of Operating Revenues
Revenues	268.0		775.3
Illinois | Public Utility | Other operating revenues
Disaggregation of Operating Revenues
Revenues	(5.9)		(6.2)
Illinois | Public Utility | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	273.9		781.5
Illinois | Public Utility | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Illinois | Public Utility | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	273.8		781.4
Illinois | Public Utility | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	273.8		781.4
Illinois | Public Utility | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.1		0.1
Other States | Public Utility
Disaggregation of Operating Revenues
Revenues	72.4		242.3
Other States | Public Utility | Other operating revenues
Disaggregation of Operating Revenues
Revenues	(0.4)		(7.1)
Other States | Public Utility | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	72.8		249.4
Other States | Public Utility | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Other States | Public Utility | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	68.9		241.6
Other States | Public Utility | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	68.9		241.6
Other States | Public Utility | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	3.9		7.8
Electric Transmission
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Electric Transmission | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Electric Transmission | Other operating revenues
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Electric Transmission | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Electric Transmission | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Electric Transmission | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Electric Transmission | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Revenues	117.0		235.1
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	19.3		41.3
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Revenues	97.7		193.8
Non-Utility Energy Infrastructure | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	10.0		24.9
Non-Utility Energy Infrastructure | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	10.0		24.9
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	9.3		16.4
Corporate and Other
Disaggregation of Operating Revenues
Revenues	3.1		4.5
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	2.8		4.1
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Revenues	0.3		0.4
Corporate and Other | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Corporate and Other | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Corporate and Other | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Corporate and Other | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	2.8		4.1
Reconciling Eliminations
Disaggregation of Operating Revenues
Revenues	(113.5)		(212.8)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	(15.8)		(19.0)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Revenues	(97.7)		(193.8)
Reconciling Eliminations | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0		0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	(12.7)		(15.2)
Reconciling Eliminations | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	(12.7)		(15.2)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	$ (3.1)		$ (3.8)