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SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended		12 Months Ended
SEGMENT INFORMATION (Details) $ in Millions	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment information
Number of reportable segments \| segment	6
Revenues	$ 2,286.5	$ 2,304.5
Other operation and maintenance	511.9	504.5	$ 2,056.1	$ 2,171.3	$ 1,710.7
Depreciation and amortization	208.6	194.6
Operating income (loss)	545.1	614.7
Equity in earnings of transmission affiliates	32.8	41.9
Interest expense	106.7	104.7
Intersegment revenues
Segment information
Revenues	0.0	0.0
External Revenues
Segment information
Revenues	2,286.5	2,304.5
Electric Transmission
Segment information
Revenues	0.0
Other operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Operating income (loss)	0.0	0.0
Equity in earnings of transmission affiliates	32.8	41.9
Interest expense	0.0	0.0
Electric Transmission \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Electric Transmission \| External Revenues
Segment information
Revenues	0.0	0.0
Non-Utility Energy Infrastructure
Segment information
Revenues	118.1
Other operation and maintenance	1.7	0.4
Depreciation and amortization	18.3	17.5
Operating income (loss)	93.0	97.4
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	16.1	15.3
Non-Utility Energy Infrastructure \| Intersegment revenues
Segment information
Revenues	99.3	109.0
Non-Utility Energy Infrastructure \| External Revenues
Segment information
Revenues	18.8	6.3
Corporate and Other
Segment information
Revenues	1.4
Other operation and maintenance	(0.3)	(0.4)
Depreciation and amortization	7.7	5.6
Operating income (loss)	(5.4)	(2.4)
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	28.0	29.1
Corporate and Other \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Corporate and Other \| External Revenues
Segment information
Revenues	1.4	2.9
Reconciling Eliminations
Segment information
Other operation and maintenance	(96.8)	(109.0)
Depreciation and amortization	0.0	0.0
Operating income (loss)	0.0	0.0
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	(1.2)	(1.8)
Reconciling Eliminations \| Intersegment revenues
Segment information
Revenues	(99.3)	(109.0)
Reconciling Eliminations \| External Revenues
Segment information
Revenues	0.0	0.0
Public Utility
Segment information
Revenues	2,266.3
Other operation and maintenance	607.3	613.5
Depreciation and amortization	182.6	171.5
Operating income (loss)	457.5	519.7
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	63.8	62.1
Public Utility \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Public Utility \| External Revenues
Segment information
Revenues	2,266.3	2,295.3
Public Utility \| Wisconsin
Segment information
Revenues	1,589.1
Other operation and maintenance	468.5	465.7
Depreciation and amortization	135.1	129.3
Operating income (loss)	273.7	329.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	49.4	48.7
Public Utility \| Wisconsin \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Public Utility \| Wisconsin \| External Revenues
Segment information
Revenues	1,589.1	1,612.1
Public Utility \| Illinois
Segment information
Revenues	507.3
Other operation and maintenance	112.2	119.6
Depreciation and amortization	40.9	36.2
Operating income (loss)	147.6	156.7
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	12.3	11.1
Public Utility \| Illinois \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Public Utility \| Illinois \| External Revenues
Segment information
Revenues	507.3	525.3
Public Utility \| Other States
Segment information
Revenues	169.9
Other operation and maintenance	26.6	28.2
Depreciation and amortization	6.6	6.0
Operating income (loss)	36.2	33.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	2.1	2.3
Public Utility \| Other States \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Public Utility \| Other States \| External Revenues
Segment information
Revenues	$ 169.9	157.9
ATC \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	60.00%
ATC Holdco
Segment information
Equity method investment, ownership interest (as a percent)	75.00%
Equity in earnings of transmission affiliates	$ (0.6)	$ (5.8)
ATC Holdco \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%