OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) contract performance_obligations	Mar. 31, 2017 USD ($)
Disaggregation of Operating Revenues
Revenues	$ 2,286.5	$ 2,304.5
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	2,290.0
Other operating revenues
Disaggregation of Operating Revenues
Revenues	$ (3.5)
Electric
Disaggregation of Operating Revenues
Number of contracts \| contract	1
Number of performance obligations \| performance_obligations	2
Number of days payment is due	30 days
Electric \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	$ 1,067.7
Natural gas
Disaggregation of Operating Revenues
Number of days payment is due	30 days
Natural gas \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	$ 1,210.7
Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,198.3
Natural gas \| Transferred over time \| Revenues from contracts with customers \| Residential
Disaggregation of Operating Revenues
Revenues	812.5
Natural gas \| Transferred over time \| Revenues from contracts with customers \| Total retail
Disaggregation of Operating Revenues
Revenues	1,174.5
Natural gas \| Transferred over time \| Revenues from contracts with customers \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues	362.0
Natural gas \| Transferred over time \| Revenues from contracts with customers \| Transport
Disaggregation of Operating Revenues
Revenues	108.6
Natural gas \| Transferred over time \| Revenues from contracts with customers \| Other utility
Disaggregation of Operating Revenues
Revenues	(84.8)
Total utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	2,278.4
Other non-utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	11.6
Public Utility
Disaggregation of Operating Revenues
Revenues	2,266.3
Public Utility \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	(3.6)
Public Utility \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	2,269.9
Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,067.7
Public Utility \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,198.3
Public Utility \| Total utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	2,266.0
Public Utility \| Other non-utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	3.9
Wisconsin \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	518.0
Wisconsin \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Residential
Disaggregation of Operating Revenues
Revenues	356.7
Wisconsin \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Total retail
Disaggregation of Operating Revenues
Revenues	544.6
Wisconsin \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues	187.9
Wisconsin \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Transport
Disaggregation of Operating Revenues
Revenues	21.0
Wisconsin \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Other utility
Disaggregation of Operating Revenues
Revenues	(47.6)
Wisconsin \| Public Utility
Disaggregation of Operating Revenues
Revenues	1,589.1
Wisconsin \| Public Utility \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	3.4
Wisconsin \| Public Utility \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,585.7
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,067.7
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Residential
Disaggregation of Operating Revenues
Revenues	384.3
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Small commercial and industrial
Disaggregation of Operating Revenues
Revenues	330.7
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Large commercial and industrial
Disaggregation of Operating Revenues
Revenues	203.9
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Other
Disaggregation of Operating Revenues
Revenues	7.7
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Total retail
Disaggregation of Operating Revenues
Revenues	926.6
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Wholesale
Disaggregation of Operating Revenues
Revenues	54.9
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Resale
Disaggregation of Operating Revenues
Revenues	73.8
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Steam
Disaggregation of Operating Revenues
Revenues	9.7
Wisconsin \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers \| Other utility
Disaggregation of Operating Revenues
Revenues	2.7
Wisconsin \| Public Utility \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	518.0
Wisconsin \| Public Utility \| Total utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1,585.7
Wisconsin \| Public Utility \| Other non-utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Illinois \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	507.6
Illinois \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Residential
Disaggregation of Operating Revenues
Revenues	332.6
Illinois \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Total retail
Disaggregation of Operating Revenues
Revenues	442.0
Illinois \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues	109.4
Illinois \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Transport
Disaggregation of Operating Revenues
Revenues	77.7
Illinois \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Other utility
Disaggregation of Operating Revenues
Revenues	(12.1)
Illinois \| Public Utility
Disaggregation of Operating Revenues
Revenues	507.3
Illinois \| Public Utility \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	(0.3)
Illinois \| Public Utility \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	507.6
Illinois \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Illinois \| Public Utility \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	507.6
Illinois \| Public Utility \| Total utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	507.6
Illinois \| Public Utility \| Other non-utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Other States \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	172.7
Other States \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Residential
Disaggregation of Operating Revenues
Revenues	123.2
Other States \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Total retail
Disaggregation of Operating Revenues
Revenues	187.9
Other States \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues	64.7
Other States \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Transport
Disaggregation of Operating Revenues
Revenues	9.9
Other States \| Natural gas \| Transferred over time \| Revenues from contracts with customers \| Other utility
Disaggregation of Operating Revenues
Revenues	(25.1)
Other States \| Public Utility
Disaggregation of Operating Revenues
Revenues	169.9
Other States \| Public Utility \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	(6.7)
Other States \| Public Utility \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	176.6
Other States \| Public Utility \| Electric \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Other States \| Public Utility \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	172.7
Other States \| Public Utility \| Total utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	172.7
Other States \| Public Utility \| Other non-utility revenues \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	3.9
Electric Transmission
Disaggregation of Operating Revenues
Revenues	0.0
Electric Transmission \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Electric Transmission \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	0.0
Electric Transmission \| Electric \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Electric Transmission \| Natural gas \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Electric Transmission \| Total utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Electric Transmission \| Other non-utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Revenues	118.1
Non-Utility Energy Infrastructure \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	22.0
Non-Utility Energy Infrastructure \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	96.1
Non-Utility Energy Infrastructure \| Electric \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Non-Utility Energy Infrastructure \| Natural gas \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	14.9
Non-Utility Energy Infrastructure \| Total utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	14.9
Non-Utility Energy Infrastructure \| Other non-utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	7.1
Corporate and Other
Disaggregation of Operating Revenues
Revenues	1.4
Corporate and Other \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1.3
Corporate and Other \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	0.1
Corporate and Other \| Electric \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Corporate and Other \| Natural gas \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Corporate and Other \| Total utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Corporate and Other \| Other non-utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	1.3
Reconciling Eliminations
Disaggregation of Operating Revenues
Revenues	(99.3)
Reconciling Eliminations \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	(3.2)
Reconciling Eliminations \| Other operating revenues
Disaggregation of Operating Revenues
Revenues	(96.1)
Reconciling Eliminations \| Electric \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	0.0
Reconciling Eliminations \| Natural gas \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	(2.5)
Reconciling Eliminations \| Total utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	(2.5)
Reconciling Eliminations \| Other non-utility revenues \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues	$ (0.7)