SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Segment information
Number of reportable segments \| segment			6
Revenues	$ 1,657.5	$ 1,712.5	$ 5,593.5	$ 5,509.3
Other operation and maintenance	471.7	517.5	1,453.4	1,571.0
Depreciation and amortization	201.2	191.6	593.5	569.5
Operating income (loss)	393.6	399.0	1,373.1	1,320.4
Equity in earnings of transmission affiliate	39.2	38.3	122.9	107.7
Interest expense	103.8	99.1	310.4	300.1
Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Electric Transmission
Segment information
Revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliate	39.2	38.3	122.9	107.7
Interest expense	0.0	0.0	0.0	0.0
Electric Transmission \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Non-Utility Energy Infrastructure
Segment information
Revenues	13.6	6.2	26.1	18.7
Other operation and maintenance	1.5	0.4	4.6	3.5
Depreciation and amortization	18.2	17.1	53.1	51.1
Operating income (loss)	103.4	93.7	299.5	281.1
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	16.2	15.6	46.7	46.8
Non-Utility Energy Infrastructure \| Intersegment revenues
Segment information
Revenues	111.6	105.0	333.2	317.1
Corporate and Other
Segment information
Revenues	5.6	4.0	11.7	20.3
Other operation and maintenance	(1.5)	(3.2)	(0.3)	(13.0)
Depreciation and amortization	6.3	11.3	18.3	33.4
Operating income (loss)	1.1	(4.5)	(6.3)	(6.4)
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	25.4	29.7	81.3	91.2
Corporate and Other \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Reconciling Eliminations
Segment information
Revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	(109.0)	(105.0)	(330.6)	(317.4)
Depreciation and amortization	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	0.4	(2.0)	(2.5)	(6.7)
Reconciling Eliminations \| Intersegment revenues
Segment information
Revenues	(111.6)	(105.0)	(333.2)	(317.4)
Public Utility
Segment information
Revenues	1,638.3	1,702.3	5,555.7	5,470.3
Other operation and maintenance	580.7	625.3	1,779.7	1,897.9
Depreciation and amortization	176.7	163.2	522.1	485.0
Operating income (loss)	289.1	309.8	1,079.9	1,045.7
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	61.8	55.8	184.9	168.8
Public Utility \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.3
Public Utility \| Wisconsin
Segment information
Revenues	1,401.3	1,470.6	4,316.6	4,354.9
Other operation and maintenance	458.3	498.2	1,379.9	1,477.3
Depreciation and amortization	131.5	124.5	391.1	370.1
Operating income (loss)	279.7	299.1	835.6	841.3
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	48.5	44.6	145.4	133.5
Public Utility \| Wisconsin \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.3
Public Utility \| Illinois
Segment information
Revenues	187.2	181.8	965.7	853.1
Other operation and maintenance	100.8	105.9	326.6	340.0
Depreciation and amortization	38.9	33.5	112.6	99.4
Operating income (loss)	12.5	11.7	209.3	171.3
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	11.0	9.3	33.0	28.8
Public Utility \| Illinois \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility \| Other States
Segment information
Revenues	49.8	49.9	273.4	262.3
Other operation and maintenance	21.6	21.2	73.2	80.6
Depreciation and amortization	6.3	5.2	18.4	15.5
Operating income (loss)	(3.1)	(1.0)	35.0	33.1
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	2.3	1.9	6.5	6.5
Public Utility \| Other States \| Intersegment revenues
Segment information
Revenues	$ 0.0	0.0	$ 0.0	0.0
ATC
Segment information
Equity method investment, ownership interest (as a percent)	60.00%		60.00%
Equity in earnings of transmission affiliate	$ 39.2	$ 38.3	$ 122.9	$ 107.7
ATC \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	60.00%		60.00%

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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