SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)
Segment information
Number of reportable segments \| segment			6
Revenues	$ 1,672.5	$ 1,631.5	$ 3,959.0	$ 3,936.0
Other operation and maintenance	537.7	479.8	1,049.6	984.3
Depreciation and amortization	206.7	197.7	415.3	392.3
Operating income (loss)	330.8	362.2	875.9	976.9
Equity in earnings of transmission affiliates	28.7	41.8	61.5	83.7
Interest expense	108.5	101.9	215.2	206.6
Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
External Revenues
Segment information
Revenues	1,672.5	1,631.5	3,959.0	3,936.0
Electric Transmission
Segment information
Revenues	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 affiliates	28.7	41.8	61.5	83.7
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
Electric Transmission \| External Revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Non-Utility Energy Infrastructure
Segment information
Revenues	117.0		235.1
Other operation and maintenance	4.5	2.7	6.2	3.1
Depreciation and amortization	18.3	17.4	36.6	34.9
Operating income (loss)	92.4	98.7	185.4	196.1
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	16.0	15.2	32.1	30.5
Non-Utility Energy Infrastructure \| Intersegment revenues
Segment information
Revenues	113.5	112.6	212.8	221.6
Non-Utility Energy Infrastructure \| External Revenues
Segment information
Revenues	3.5	6.2	22.3	12.5
Corporate and Other
Segment information
Revenues	3.1		4.5
Other operation and maintenance	2.2	4.3	1.9	3.9
Depreciation and amortization	7.5	6.4	15.2	12.0
Operating income (loss)	(6.5)	(7.7)	(11.9)	(10.1)
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	30.3	26.8	58.3	55.9
Corporate and Other \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Corporate and Other \| External Revenues
Segment information
Revenues	3.1	3.2	4.5	6.1
Reconciling Eliminations
Segment information
Other operation and maintenance	(100.4)	(112.6)	(197.2)	(221.6)
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 affiliates	0.0	0.0	0.0	0.0
Interest expense	(0.7)	(1.1)	(1.9)	(2.9)
Reconciling Eliminations \| Intersegment revenues
Segment information
Revenues	(113.5)	(112.6)	(212.8)	(221.6)
Reconciling Eliminations \| External Revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility
Segment information
Revenues	1,665.9		3,932.2
Other operation and maintenance	631.4	585.4	1,238.7	1,198.9
Depreciation and amortization	180.9	173.9	363.5	345.4
Operating income (loss)	244.9	271.2	702.4	790.9
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	62.9	61.0	126.7	123.1
Public Utility \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility \| External Revenues
Segment information
Revenues	1,665.9	1,622.1	3,932.2	3,917.4
Public Utility \| Wisconsin
Segment information
Revenues	1,325.5		2,914.6
Other operation and maintenance	502.4	459.7	970.9	925.4
Depreciation and amortization	134.6	130.3	269.7	259.6
Operating income (loss)	195.1	222.6	468.8	552.1
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	48.5	48.2	97.9	96.9
Public Utility \| Wisconsin \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility \| Wisconsin \| External Revenues
Segment information
Revenues	1,325.5	1,303.2	2,914.6	2,915.3
Public Utility \| Illinois
Segment information
Revenues	268.0		775.3
Other operation and maintenance	104.1	102.4	216.3	222.0
Depreciation and amortization	41.8	37.5	82.7	73.7
Operating income (loss)	41.7	43.9	189.3	200.6
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	12.3	10.9	24.6	22.0
Public Utility \| Illinois \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility \| Illinois \| External Revenues
Segment information
Revenues	268.0	253.2	775.3	778.5
Public Utility \| Other States
Segment information
Revenues	72.4		242.3
Other operation and maintenance	24.9	23.3	51.5	51.5
Depreciation and amortization	4.5	6.1	11.1	12.1
Operating income (loss)	8.1	4.7	44.3	38.2
Equity in earnings of transmission affiliates	0.0	0.0	0.0	0.0
Interest expense	2.1	1.9	4.2	4.2
Public Utility \| Other States \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility \| Other States \| External Revenues
Segment information
Revenues	$ 72.4	65.7	$ 242.3	223.6
ATC \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	60.00%		60.00%
ATC Holdco
Segment information
Equity method investment, ownership interest (as a percent)	75.00%		75.00%
Equity in earnings of transmission affiliates	$ (1.1)	$ (1.0)	$ (1.7)	$ (6.8)
ATC Holdco \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%		75.00%

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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