SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)
Segment information
Number of reportable segments \| segment	6
Revenues	$ 2,304.5	$ 2,194.8
Other operation and maintenance	501.9	531.5
Depreciation and amortization	194.6	187.9
Operating income (loss)	617.3	589.3
Equity in earnings of transmission affiliate	41.9	38.5
Interest expense	104.7	100.9
Intersegment revenues
Segment information
Revenues	0.0	0.0
We Power
Segment information
Revenues	6.3	6.2
Other operation and maintenance	0.4	0.4
Depreciation and amortization	17.5	17.0
Operating income (loss)	97.4	93.3
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	15.3	15.6
We Power \| Intersegment revenues
Segment information
Revenues	109.0	104.5
Corporate and Other
Segment information
Revenues	2.9	11.9
Other operation and maintenance	(1.6)	(3.5)
Depreciation and amortization	5.6	10.1
Operating income (loss)	(1.2)	(0.3)
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	29.1	31.3
Corporate and Other \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Reconciling Eliminations
Segment information
Revenues	0.0	0.0
Other operation and maintenance	(109.0)	(104.6)
Depreciation and amortization	0.0	0.0
Operating income (loss)	0.0	0.0
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	(1.8)	(2.7)
Reconciling Eliminations \| Intersegment revenues
Segment information
Revenues	(109.0)	(104.6)
Regulated operations
Segment information
Revenues	2,295.3	2,176.7
Other operation and maintenance	612.1	639.2
Depreciation and amortization	171.5	160.8
Operating income (loss)	521.1	496.3
Equity in earnings of transmission affiliate	41.9	38.5
Interest expense	62.1	56.7
Regulated operations \| Intersegment revenues
Segment information
Revenues	0.0	0.1
Regulated operations \| Wisconsin
Segment information
Revenues	1,612.1	1,579.8
Other operation and maintenance	462.9	491.3
Depreciation and amortization	129.3	122.9
Operating income (loss)	332.3	327.5
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	48.7	44.5
Regulated operations \| Wisconsin \| Intersegment revenues
Segment information
Revenues	0.0	0.1
Regulated operations \| Illinois
Segment information
Revenues	525.3	448.5
Other operation and maintenance	120.9	117.9
Depreciation and amortization	36.2	32.8
Operating income (loss)	155.4	137.0
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	11.1	9.7
Regulated operations \| Illinois \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Regulated operations \| Other States
Segment information
Revenues	157.9	148.4
Other operation and maintenance	28.3	30.0
Depreciation and amortization	6.0	5.1
Operating income (loss)	33.4	31.8
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	2.3	2.5
Regulated operations \| Other States \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Regulated operations \| Electric Transmission
Segment information
Revenues	0.0	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 affiliate	41.9	38.5
Interest expense	0.0	0.0
Regulated operations \| Electric Transmission \| Intersegment revenues
Segment information
Revenues	$ 0.0	0.0
ATC
Segment information
Equity method investment, ownership interest (as a percent)	60.00%
Equity in earnings of transmission affiliate	$ 41.9	$ 38.5

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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