OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (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
Natural gas
Disaggregation of Operating Revenues
Number of days payment is due			30 days
Public Utility
Disaggregation of Operating Revenues
Revenues	1,665.9		$ 3,932.2
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,673.6		3,963.6
Revenues from contracts with customers | Natural gas
Disaggregation of Operating Revenues
Revenues	576.4		1,787.1
Revenues from contracts with customers | Public Utility | Transferred over time
Disaggregation of Operating Revenues
Revenues	1,667.3		3,937.2
Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues	579.1		1,777.4
Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues	329.7		1,142.2
Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues	129.5		491.5
Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues	459.2		1,633.7
Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues	77.8		186.4
Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues	42.1		(42.7)
Wisconsin | Public Utility
Disaggregation of Operating Revenues
Revenues	1,325.5		2,914.6
Wisconsin | Revenues from contracts with customers | Public Utility | Transferred over time
Disaggregation of Operating Revenues
Revenues	1,320.6		2,906.3
Wisconsin | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues	236.4		754.4
Wisconsin | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues	128.1		484.8
Wisconsin | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues	63.5		251.4
Wisconsin | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues	191.6		736.2
Wisconsin | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues	16.4		37.4
Wisconsin | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues	28.4		(19.2)
Illinois | Public Utility
Disaggregation of Operating Revenues
Revenues	268.0		775.3
Illinois | Revenues from contracts with customers | Public Utility | Transferred over time
Disaggregation of Operating Revenues
Revenues	273.9		781.5
Illinois | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues	273.8		781.4
Illinois | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues	163.7		496.3
Illinois | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues	47.3		156.7
Illinois | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues	211.0		653.0
Illinois | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues	54.6		132.3
Illinois | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues	8.2		(3.9)
Other States | Public Utility
Disaggregation of Operating Revenues
Revenues	72.4		242.3
Other States | Revenues from contracts with customers | Public Utility | Transferred over time
Disaggregation of Operating Revenues
Revenues	72.8		249.4
Other States | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues	68.9		241.6
Other States | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues	37.9		161.1
Other States | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues	18.7		83.4
Other States | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues	56.6		244.5
Other States | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues	6.8		16.7
Other States | Revenues from contracts with customers | Public Utility | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues	$ 5.5		$ (19.6)