SEGMENTS OF BUSINESS (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Apr. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 segment	Sep. 30, 2013	Sep. 30, 2014 ATC	Sep. 30, 2013 ATC	Sep. 30, 2014 ATC	Sep. 30, 2013 ATC	Sep. 30, 2014 Intersegment revenues	Sep. 30, 2013 Intersegment revenues	Sep. 30, 2014 Intersegment revenues	Sep. 30, 2013 Intersegment revenues	Sep. 30, 2014 Natural Gas Utility	Jun. 30, 2014 IES	Sep. 30, 2014 IES	Sep. 30, 2014 Holding Company and Other	Sep. 30, 2013 Holding Company and Other	Sep. 30, 2014 Reconciling Eliminations	Sep. 30, 2013 Reconciling Eliminations	Sep. 30, 2014 Reconciling Eliminations	Sep. 30, 2013 Reconciling Eliminations	Sep. 30, 2014 Reconciling Eliminations Intersegment revenues	Sep. 30, 2013 Reconciling Eliminations Intersegment revenues	Sep. 30, 2014 Reconciling Eliminations Intersegment revenues	Sep. 30, 2013 Reconciling Eliminations Intersegment revenues	Sep. 30, 2014 Regulated operations	Sep. 30, 2013 Regulated operations	Sep. 30, 2014 Regulated operations	Sep. 30, 2013 Regulated operations	Sep. 30, 2014 Regulated operations Intersegment revenues	Sep. 30, 2013 Regulated operations Intersegment revenues	Sep. 30, 2014 Regulated operations Intersegment revenues	Sep. 30, 2013 Regulated operations Intersegment revenues	Sep. 30, 2014 Regulated operations Natural Gas Utility	Sep. 30, 2013 Regulated operations Natural Gas Utility	Sep. 30, 2014 Regulated operations Natural Gas Utility	Sep. 30, 2013 Regulated operations Natural Gas Utility	Sep. 30, 2014 Regulated operations Natural Gas Utility Intersegment revenues	Sep. 30, 2013 Regulated operations Natural Gas Utility Intersegment revenues	Sep. 30, 2014 Regulated operations Natural Gas Utility Intersegment revenues	Sep. 30, 2013 Regulated operations Natural Gas Utility Intersegment revenues	Sep. 30, 2014 Regulated operations Electric Utility	Sep. 30, 2013 Regulated operations Electric Utility	Sep. 30, 2014 Regulated operations Electric Utility	Sep. 30, 2013 Regulated operations Electric Utility	Sep. 30, 2014 Regulated operations Electric Utility Intersegment revenues	Sep. 30, 2013 Regulated operations Electric Utility Intersegment revenues	Sep. 30, 2014 Regulated operations Electric Utility Intersegment revenues	Sep. 30, 2013 Regulated operations Electric Utility Intersegment revenues	Sep. 30, 2014 Regulated operations Electric Transmission Investment	Sep. 30, 2013 Regulated operations Electric Transmission Investment	Sep. 30, 2014 Regulated operations Electric Transmission Investment	Sep. 30, 2013 Regulated operations Electric Transmission Investment	Sep. 30, 2014 Regulated operations Electric Transmission Investment Intersegment revenues	Sep. 30, 2013 Regulated operations Electric Transmission Investment Intersegment revenues	Sep. 30, 2014 Regulated operations Electric Transmission Investment Intersegment revenues	Sep. 30, 2013 Regulated operations Electric Transmission Investment Intersegment revenues	Sep. 30, 2014 Nonutility and nonregulated operations IES	Sep. 30, 2013 Nonutility and nonregulated operations IES	Sep. 30, 2014 Nonutility and nonregulated operations IES	Sep. 30, 2013 Nonutility and nonregulated operations IES	Sep. 30, 2014 Nonutility and nonregulated operations IES Intersegment revenues	Sep. 30, 2013 Nonutility and nonregulated operations IES Intersegment revenues	Sep. 30, 2014 Nonutility and nonregulated operations IES Intersegment revenues	Sep. 30, 2013 Nonutility and nonregulated operations IES Intersegment revenues	Sep. 30, 2014 Nonutility and nonregulated operations Holding Company and Other	Sep. 30, 2013 Nonutility and nonregulated operations Holding Company and Other	Sep. 30, 2014 Nonutility and nonregulated operations Holding Company and Other	Sep. 30, 2013 Nonutility and nonregulated operations Holding Company and Other	Sep. 30, 2014 Nonutility and nonregulated operations Holding Company and Other Intersegment revenues	Sep. 30, 2013 Nonutility and nonregulated operations Holding Company and Other Intersegment revenues	Sep. 30, 2014 Nonutility and nonregulated operations Holding Company and Other Intersegment revenues	Sep. 30, 2013 Nonutility and nonregulated operations Holding Company and Other Intersegment revenues
Segment reporting information
Number of reportable segments				5
Equity method investment, ownership interest (as a percent)						34.00%		34.00%
Revenues		$ 1,187.9	$ 1,129.7	$ 5,545.4	$ 3,923.9					$ 0	$ 0	$ 0	$ 0						$ 0	$ 0	$ 0	$ 0	$ (4.6)	$ (4.9)	$ (15.6)	$ (10.6)	$ 625.1	$ 606.9	$ 3,047.9	$ 2,425.1	$ 3.8	$ 4.3	$ 11.1	$ 8.7	$ 282.7	$ 253.0	$ 2,043.7	$ 1,412.4	$ 3.7	$ 4.2	$ 11.0	$ 8.6	$ 342.4	$ 353.9	$ 1,004.2	$ 1,012.7	$ 0.1	$ 0.1	$ 0.1	$ 0.1	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 537.0	$ 512.7	$ 2,426.6	$ 1,470.7	$ 0.4	$ 0.3	$ 3.4	$ 0.9	$ 25.8	$ 10.1	$ 70.9	$ 28.1	$ 0.4	$ 0.3	$ 1.1	$ 1.0
Depreciation and amortization expense		73.3	69.6	217.5	196.0														(0.1)	(0.1)	(0.4)	(0.4)					63.3	61.3	188.5	173.1					37.3	35.6	110.6	100.1					26.0	25.7	77.9	73.0					0	0	0	0					3.1	2.9	9.0	8.4					7.0	5.5	20.4	14.9
Merger transaction costs		2.5	0	8.4	0														0		0						0		0						0		0						0		0						0		0						0		0						2.5		8.4
Goodwill impairment loss	0	0	0	6.7	0									0	6.7	6.7	0				0								0								0								0								0								6.7								0
Transaction costs related to sale of IES's retail energy business		0.9	0	1.7	0														0		0						0		0						0		0						0		0						0		0						0.9		1.7						0		0
Gain on sale of UPPCO, net of transaction costs		(86.3)	0	(85.4)	0														0		0						(86.3)		(85.4)						0		0						(86.3)		(85.4)						0		0						0		0						0		0
Gain on abandonment of IES's Winnebago Energy Center		(4.1)	0	(4.1)	0														0		0						0		0						0		0						0		0						0		0						(4.1)		(4.1)						0		0
Earnings from equity method investments		24.5	23.1	71.3	68.2	23.4	22.3	68.9	66.0										0	0	0	0					23.4	22.3	68.9	66.0					0	0	0	0					0	0	0	0					23.4	22.3	68.9	66.0					0.7	0.5	1.6	1.2					0.4	0.3	0.8	1.0
Miscellaneous income		6.4	12.1	17.4	23.3														(2.9)	(3.0)	(9.6)	(10.1)					3.4	3.2	9.6	7.4					1.3	0.4	1.2	0.8					2.1	2.8	8.4	6.6					0	0	0	0					0.3	6.2	1.0	8.0					5.6	5.7	16.4	18.0
Interest expense		38.1	33.1	115.9	91.0														(2.9)	(3.0)	(9.6)	(10.1)					25.4	21.5	75.8	63.7					13.4	12.7	40.0	37.3					12.0	8.8	35.8	26.4					0	0	0	0					0.5	0.5	1.5	1.5					15.1	14.1	48.2	35.9
Provision (benefit) for income taxes		56.8	18.0	154.8	124.3														0	0	0	0					52.1	14.2	158.2	125.3					(20.1)	(19.5)	39.3	43.1					63.0	25.1	91.7	56.9					9.2	8.6	27.2	25.3					6.9	6.6	14.9	11.7					(2.2)	(2.8)	(18.3)	(12.7)
Net income (loss) from continuing operations		82.9	39.4	244.2	217.7														0	0	0	0					81.8	35.1	247.1	207.2					(29.5)	(19.5)	59.2	72.0					97.1	40.9	146.2	94.5					14.2	13.7	41.7	40.7					11.1	12.3	20.0	22.5					(10.0)	(8.0)	(22.9)	(12.0)
Discontinued operations, net of tax		1.1	(0.6)	0.9	4.7													5.9	0	0	0	0					0	0	0	0					0	0	0	0					0	0	0	0					0	0	0	0					1.1	(0.6)	0.9	(1.2)					0	0	0	5.9
Preferred stock dividends of subsidiary		(0.7)	(0.7)	(2.3)	(2.3)														0	0	0	0					(0.7)	(0.7)	(2.3)	(2.3)					(0.1)	(0.1)	(0.3)	(0.4)					(0.6)	(0.6)	(2.0)	(1.9)					0	0	0	0					0	0	0	0					0	0	0	0
Noncontrolling interest in subsidiaries		0	0	0.1	0.1																0	0							0	0							0	0							0	0							0	0							0	0							0.1	0.1
Net income attributed to common shareholders		$ 83.3	$ 38.1	$ 242.9	$ 220.2														$ 0	$ 0	$ 0	$ 0					$ 81.1	$ 34.4	$ 244.8	$ 204.9					$ (29.6)	$ (19.6)	$ 58.9	$ 71.6					$ 96.5	$ 40.3	$ 144.2	$ 92.6					$ 14.2	$ 13.7	$ 41.7	$ 40.7					$ 12.2	$ 11.7	$ 20.9	$ 21.3					$ (10.0)	$ (8.0)	$ (22.8)	$ (6.0)