SEGMENTS OF BUSINESS (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)
Segment reporting information
Number of reportable segments | segment			5
Revenues	$ 638.3	$ 837.1	$ 1,801.8	$ 2,475.4
Depreciation and amortization expense	72.1	72.8	147.5	144.2
Merger costs	59.3	5.9	60.0	5.9
Impairment loss on property, plant, and equipment at PDL	10.7	0.0	10.7	0.0
Gain on sale of certain PDL solar power generation plants	(5.2)	0.0	(5.2)	0.0
Equity in earnings of transmission affiliate	22.4	23.0	39.4	45.5
Miscellaneous income (expense)	1.8	5.8	9.1	12.3
Interest expense	40.0	39.3	79.0	79.0
Provision (benefit) for income taxes	(8.1)	4.5	70.9	86.4
Net income (loss) from continuing operations	(9.3)	8.0	121.0	147.5
Discontinued operations	0.0	(0.8)	(0.8)	12.1
Net income (loss)	(9.3)	7.2	120.2	159.6
Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
Reconciling Eliminations
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Depreciation and amortization expense	0.0	0.0	0.0	0.0
Merger costs	0.0	0.0	0.0	0.0
Impairment loss on property, plant, and equipment at PDL	0.0		0.0
Gain on sale of certain PDL solar power generation plants	0.0		0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Miscellaneous income (expense)	(2.5)	(3.1)	(4.5)	(6.5)
Interest expense	(2.5)	(3.1)	(4.5)	(6.5)
Provision (benefit) for income taxes	0.0	0.0	0.0	0.0
Net income (loss) from continuing operations		0.0	0.0	0.0
Discontinued operations		0.0	0.0	0.0
Net income (loss)	0.0	0.0	0.0	0.0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues		(5.8)		(11.2)
Regulated operations
Segment reporting information
Revenues	616.7	810.6	1,755.9	2,431.2
Depreciation and amortization expense	65.8	64.0	132.1	126.9
Merger costs	0.0	0.0	0.0	0.0
Impairment loss on property, plant, and equipment at PDL	0.0		0.0
Gain on sale of certain PDL solar power generation plants	0.0		0.0
Equity in earnings of transmission affiliate	22.4	23.0	39.4	45.5
Miscellaneous income (expense)	4.5	3.7	9.7	9.0
Interest expense	27.1	26.4	53.6	52.9
Provision (benefit) for income taxes	26.1	13.1	115.3	106.4
Net income (loss) from continuing operations		21.3	176.9	166.1
Discontinued operations		0.0	0.0	0.0
Net income (loss)	40.1	21.3	176.9	166.1
Regulated operations | Intersegment revenues
Segment reporting information
Revenues		5.8		11.2
Regulated operations | Wisconsin
Segment reporting information
Revenues	330.2	353.1	755.3	903.7
Depreciation and amortization expense	30.2	29.0	60.1	57.4
Merger costs	0.0	0.0	0.0	0.0
Impairment loss on property, plant, and equipment at PDL	0.0		0.0
Gain on sale of certain PDL solar power generation plants	0.0		0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Miscellaneous income (expense)	4.3	4.2	8.8	9.2
Interest expense	14.5	15.1	29.2	29.9
Provision (benefit) for income taxes	13.0	9.9	35.1	38.7
Net income (loss) from continuing operations		15.5	58.6	64.3
Discontinued operations		0.0	0.0	0.0
Net income (loss)	21.0	15.5	58.6	64.3
Regulated operations | Wisconsin | Intersegment revenues
Segment reporting information
Revenues		5.8		11.2
Regulated operations | Illinois
Segment reporting information
Revenues	218.6	351.0	750.2	1,127.8
Depreciation and amortization expense	30.8	28.4	62.5	56.4
Merger costs	0.0	0.0	0.0	0.0
Impairment loss on property, plant, and equipment at PDL	0.0		0.0
Gain on sale of certain PDL solar power generation plants	0.0		0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Miscellaneous income (expense)	0.2	(0.5)	0.9	(0.2)
Interest expense	9.9	8.2	19.4	16.5
Provision (benefit) for income taxes	3.6	(3.8)	52.3	35.6
Net income (loss) from continuing operations		(5.4)	76.2	52.3
Discontinued operations		0.0	0.0	0.0
Net income (loss)	4.7	(5.4)	76.2	52.3
Regulated operations | Illinois | Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
Regulated operations | Other States
Segment reporting information
Revenues	67.9	106.5	250.4	399.7
Depreciation and amortization expense	4.8	6.6	9.5	13.1
Merger costs	0.0	0.0	0.0	0.0
Impairment loss on property, plant, and equipment at PDL	0.0		0.0
Gain on sale of certain PDL solar power generation plants	0.0		0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Miscellaneous income (expense)	0.0	0.0	0.0	0.0
Interest expense	2.7	3.1	5.0	6.5
Provision (benefit) for income taxes	0.6	(2.2)	12.3	14.1
Net income (loss) from continuing operations		(2.6)	18.3	22.0
Discontinued operations		0.0	0.0	0.0
Net income (loss)	0.9	(2.6)	18.3	22.0
Regulated operations | Other States | Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
Regulated operations | Electric Transmission Investment
Segment reporting information
Revenues	0.0	0.0	0.0	0.0
Depreciation and amortization expense	0.0	0.0	0.0	0.0
Merger costs	0.0	0.0	0.0	0.0
Impairment loss on property, plant, and equipment at PDL	0.0		0.0
Gain on sale of certain PDL solar power generation plants	0.0		0.0
Equity in earnings of transmission affiliate	22.4	23.0	39.4	45.5
Miscellaneous income (expense)	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Provision (benefit) for income taxes	8.9	9.2	15.6	18.0
Net income (loss) from continuing operations		13.8	23.8	27.5
Discontinued operations		0.0	0.0	0.0
Net income (loss)	13.5	13.8	23.8	27.5
Regulated operations | Electric Transmission Investment | Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
Nonregulated operations | Corporate and Other
Segment reporting information
Revenues	21.6	26.5	45.9	44.2
Depreciation and amortization expense	6.3	8.8	15.4	17.3
Merger costs	59.3	5.9	60.0	5.9
Impairment loss on property, plant, and equipment at PDL	10.7		10.7
Gain on sale of certain PDL solar power generation plants	(5.2)		(5.2)
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Miscellaneous income (expense)	(0.2)	5.2	3.9	9.8
Interest expense	15.4	16.0	29.9	32.6
Provision (benefit) for income taxes	(34.2)	(8.6)	(44.4)	(20.0)
Net income (loss) from continuing operations		(13.3)	(55.9)	(18.6)
Discontinued operations		(0.8)	(0.8)	12.1
Net income (loss)	$ (49.4)	(14.1)	$ (56.7)	(6.5)
Nonregulated operations | Corporate and Other | Intersegment revenues
Segment reporting information
Revenues		0.0		0.0
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)	34.00%		34.00%
Equity in earnings of transmission affiliate	$ 22.4	$ 23.0	$ 39.4	$ 45.5