Segment Information (Details) $ in Millions	3 Months Ended
Mar. 31, 2016 USD ($) segment	Mar. 31, 2015 USD ($)	Jun. 29, 2015
Segment information
Number of reportable segments \| segment	6
Revenues	$ 2,194.8	$ 1,387.9
Other operation and maintenance	531.5	280.7
Depreciation and amortization	187.9	102.6
Operating income (loss)	589.3	358.8
Equity in earnings of transmission affiliate	38.5	16.1
Interest expense	100.9	59.4
Intersegment revenues
Segment information
Revenues	0.0	0.0
We Power
Segment information
Revenues	6.2	11.1
Other operation and maintenance	0.4	0.4
Depreciation and amortization	17.0	16.8
Operating income (loss)	93.3	92.5
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	15.6	15.9
We Power \| Intersegment revenues
Segment information
Revenues	104.5	98.6
Corporate and other
Segment information
Revenues	11.9	0.3
Other operation and maintenance	(3.5)	10.1
Depreciation and amortization	10.1	0.4
Operating income (loss)	(0.3)	(10.2)
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	31.3	12.2
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	(104.6)	(98.9)
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	(2.7)	(0.1)
Reconciling eliminations \| Intersegment revenues
Segment information
Revenues	(104.6)	(99.0)
Regulated operations
Segment information
Revenues	2,176.7	1,376.5
Other operation and maintenance	639.2	369.1
Depreciation and amortization	160.8	85.4
Operating income (loss)	496.3	276.5
Equity in earnings of transmission affiliate	38.5	16.1
Interest expense	56.7	31.4
Regulated operations \| Intersegment revenues
Segment information
Revenues	0.1	0.4
Regulated operations \| Wisconsin
Segment information
Revenues	1,579.8	1,376.5
Other operation and maintenance	491.3	369.1
Depreciation and amortization	122.9	85.4
Operating income (loss)	327.5	276.5
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	44.5	31.4
Regulated operations \| Wisconsin \| Intersegment revenues
Segment information
Revenues	0.1	0.4
Regulated operations \| Illinois
Segment information
Revenues	448.5	0.0
Other operation and maintenance	117.9	0.0
Depreciation and amortization	32.8	0.0
Operating income (loss)	137.0	0.0
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	9.7	0.0
Regulated operations \| Illinois \| Intersegment revenues
Segment information
Revenues	0.0	0.0
Regulated operations \| Other States
Segment information
Revenues	148.4	0.0
Other operation and maintenance	30.0	0.0
Depreciation and amortization	5.1	0.0
Operating income (loss)	31.8	0.0
Equity in earnings of transmission affiliate	0.0	0.0
Interest expense	2.5	0.0
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	38.5	16.1
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%		26.20%
Equity in earnings of transmission affiliate	$ 38.5	$ 16.1

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