SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment reporting information
Revenues	$ 1,163.2us-gaap_Revenues	$ 1,638.0us-gaap_Revenues
Depreciation and amortization expense	74.7	70.6
Earnings from equity method investments	17.5	22.9
Miscellaneous income	6.6us-gaap_OtherNonoperatingIncomeExpense	5.8us-gaap_OtherNonoperatingIncomeExpense
Interest expense	38.2us-gaap_InterestExpense	38.9us-gaap_InterestExpense
Provision (benefit) for income taxes	78.9us-gaap_IncomeTaxExpenseBenefit	81.9us-gaap_IncomeTaxExpenseBenefit
Net income (loss) from continuing operations	131.2	140.2
Discontinued operations	(0.9)	12.9
Preferred stock dividends of subsidiary	(0.8)us-gaap_DividendsPreferredStockCash	(0.8)us-gaap_DividendsPreferredStockCash
Noncontrolling interest in subsidiaries	0	0.1
Net income attributed to common shareholders	129.5	152.4
Intersegment revenues
Segment reporting information
Revenues	0	0
Reconciling Eliminations
Segment reporting information
Revenues	0	0
Depreciation and amortization expense	(0.1)	(0.1)
Earnings from equity method investments	0	0
Miscellaneous income	(2.0)	(3.3)
Interest expense	(2.0)	(3.3)
Provision (benefit) for income taxes	0	0
Net income (loss) from continuing operations	0	0
Discontinued operations	0	0
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries		0
Net income attributed to common shareholders	0	0
Reconciling Eliminations \| Intersegment revenues
Segment reporting information
Revenues	(2.8)	(4.8)
Regulated operations
Segment reporting information
Revenues	1,135.3	1,616.8
Depreciation and amortization expense	65.6	62.0
Earnings from equity method investments	17.0	22.5
Miscellaneous income	3.5	3.8
Interest expense	25.2	25.1
Provision (benefit) for income taxes	89.0	93.6
Net income (loss) from continuing operations	136.7	144.7
Discontinued operations	0	0
Preferred stock dividends of subsidiary	(0.8)	(0.8)
Noncontrolling interest in subsidiaries		0
Net income attributed to common shareholders	135.9	143.9
Regulated operations \| Intersegment revenues
Segment reporting information
Revenues	2.6	4.4
Regulated operations \| Natural Gas Utility
Segment reporting information
Revenues	839.5	1,267.6
Depreciation and amortization expense	40.6	36.4
Earnings from equity method investments	0	0
Miscellaneous income	0.7	0.3
Interest expense	14.3	13.4
Provision (benefit) for income taxes	65.9	66.7
Net income (loss) from continuing operations	97.8	99.2
Discontinued operations	0	0
Preferred stock dividends of subsidiary	(0.1)	(0.1)
Noncontrolling interest in subsidiaries		0
Net income attributed to common shareholders	97.7	99.1
Regulated operations \| Natural Gas Utility \| Intersegment revenues
Segment reporting information
Revenues	2.6	4.4
Regulated operations \| Electric Utility
Segment reporting information
Revenues	295.8	349.2
Depreciation and amortization expense	25.0	25.6
Earnings from equity method investments	0	0
Miscellaneous income	2.8	3.5
Interest expense	10.9	11.7
Provision (benefit) for income taxes	16.4	18.1
Net income (loss) from continuing operations	28.6	31.8
Discontinued operations	0	0
Preferred stock dividends of subsidiary	(0.7)	(0.7)
Noncontrolling interest in subsidiaries		0
Net income attributed to common shareholders	27.9	31.1
Regulated operations \| Electric Utility \| Intersegment revenues
Segment reporting information
Revenues	0	0
Regulated operations \| Electric Transmission Investment
Segment reporting information
Revenues	0	0
Depreciation and amortization expense	0	0
Earnings from equity method investments	17.0	22.5
Miscellaneous income	0	0
Interest expense	0	0
Provision (benefit) for income taxes	6.7	8.8
Net income (loss) from continuing operations	10.3	13.7
Discontinued operations	0	0
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries		0
Net income attributed to common shareholders	10.3	13.7
Regulated operations \| Electric Transmission Investment \| Intersegment revenues
Segment reporting information
Revenues	0	0
Nonutility and nonregulated operations \| IES
Segment reporting information
Revenues	0	0
Depreciation and amortization expense	0	0
Earnings from equity method investments	0	0
Miscellaneous income	0	0
Interest expense	0	0
Provision (benefit) for income taxes	0	0
Net income (loss) from continuing operations	0	0
Discontinued operations	(0.8)	13.0
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries		0
Net income attributed to common shareholders	(0.8)	13.0
Nonutility and nonregulated operations \| IES \| Intersegment revenues
Segment reporting information
Revenues	0	0
Nonutility and nonregulated operations \| Holding Company and Other
Segment reporting information
Revenues	27.9	21.2
Depreciation and amortization expense	9.2	8.7
Earnings from equity method investments	0.5	0.4
Miscellaneous income	5.1	5.3
Interest expense	15.0	17.1
Provision (benefit) for income taxes	(10.1)	(11.7)
Net income (loss) from continuing operations	(5.5)	(4.5)
Discontinued operations	(0.1)	(0.1)
Preferred stock dividends of subsidiary	0	0
Noncontrolling interest in subsidiaries		0.1
Net income attributed to common shareholders	(5.6)	(4.5)
Nonutility and nonregulated operations \| Holding Company and Other \| Intersegment revenues
Segment reporting information
Revenues	0.2	0.4
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)	34.00%
Earnings from equity method investments	$ 17.0	$ 22.5
Continuing operations
Segment reporting information
Number of reportable segments	4
Discontinued operations
Segment reporting information
Number of reportable segments	1

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