SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment information
Number of reportable segments \| segment			6
Revenues	$ 1,631.5	$ 1,602.0	$ 3,936.0	$ 3,796.8
Other operation and maintenance	479.8	522.0	981.7	1,053.5
Depreciation and amortization	197.7	190.0	392.3	377.9
Operating income (loss)	362.2	332.1	979.5	921.4
Equity in earnings of transmission affiliate	41.8	30.9	83.7	69.4
Interest expense	101.9	100.1	206.6	201.0
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	41.8	30.9	83.7	69.4
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
Segment information
Revenues	6.2	6.3	12.5	12.5
Other operation and maintenance	2.7	2.7	3.1	3.1
Depreciation and amortization	17.4	17.0	34.9	34.0
Operating income (loss)	98.7	94.1	196.1	187.4
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	15.2	15.6	30.5	31.2
Non-Utility Energy \| Intersegment revenues
Segment information
Revenues	112.6	107.6	221.6	212.1
Corporate and Other
Segment information
Revenues	3.2	4.4	6.1	16.3
Other operation and maintenance	2.8	(6.3)	1.2	(9.8)
Depreciation and amortization	6.4	12.0	12.0	22.1
Operating income (loss)	(6.2)	(1.6)	(7.4)	(1.9)
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	26.8	30.2	55.9	61.5
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	(112.6)	(107.8)	(221.6)	(212.4)
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	(1.1)	(2.0)	(2.9)	(4.7)
Reconciling Eliminations \| Intersegment revenues
Segment information
Revenues	(112.6)	(107.8)	(221.6)	(212.4)
Public Utility
Segment information
Revenues	1,622.1	1,591.3	3,917.4	3,768.0
Other operation and maintenance	586.9	633.4	1,199.0	1,272.6
Depreciation and amortization	173.9	161.0	345.4	321.8
Operating income (loss)	269.7	239.6	790.8	735.9
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	61.0	56.3	123.1	113.0
Public Utility \| Intersegment revenues
Segment information
Revenues	0.0	0.2	0.0	0.3
Public Utility \| Wisconsin
Segment information
Revenues	1,303.2	1,304.5	2,915.3	2,884.3
Other operation and maintenance	458.7	487.8	921.6	979.1
Depreciation and amortization	130.3	122.7	259.6	245.6
Operating income (loss)	223.6	214.7	555.9	542.2
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	48.2	44.4	96.9	88.9
Public Utility \| Wisconsin \| Intersegment revenues
Segment information
Revenues	0.0	0.2	0.0	0.3
Public Utility \| Illinois
Segment information
Revenues	253.2	222.8	778.5	671.3
Other operation and maintenance	104.9	116.2	225.8	234.1
Depreciation and amortization	37.5	33.1	73.7	65.9
Operating income (loss)	41.4	22.6	196.8	159.6
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	10.9	9.8	22.0	19.5
Public Utility \| Illinois \| Intersegment revenues
Segment information
Revenues	0.0	0.0	0.0	0.0
Public Utility \| Other States
Segment information
Revenues	65.7	64.0	223.6	212.4
Other operation and maintenance	23.3	29.4	51.6	59.4
Depreciation and amortization	6.1	5.2	12.1	10.3
Operating income (loss)	4.7	2.3	38.1	34.1
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	1.9	2.1	4.2	4.6
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	$ 41.8	$ 30.9	$ 83.7	$ 69.4
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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