CONDENSED CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Utility revenues	$ 625.1	$ 606.9	$ 3,047.9	$ 2,425.1
Nonregulated revenues	562.8	522.8	2,497.5	1,498.8
Total revenues	1,187.9	1,129.7	5,545.4	3,923.9
Utility cost of fuel, natural gas, and purchased power	228.6	222.8	1,571.8	1,083.9
Nonregulated cost of sales	510.0	475.3	2,334.0	1,360.0
Operating and maintenance expense	289.8	282.3	988.7	866.1
Depreciation and amortization expense	73.3	69.6	217.5	196.0
Taxes other than income taxes	26.3	24.4	79.9	76.4
Merger transaction costs	2.5	0	8.4	0
Goodwill impairment loss	0	0	6.7	0
Transaction costs related to sale of IES's retail energy business	0.9	0	1.7	0
Gain on sale of UPPCO, net of transaction costs	(86.3)	0	(85.4)	0
Gain on abandonment of IES's Winnebago Energy Center	(4.1)	0	(4.1)	0
Operating income	146.9	55.3	426.2	341.5
Earnings from equity method investments	24.5	23.1	71.3	68.2
Miscellaneous income	6.4	12.1	17.4	23.3
Interest expense	38.1	33.1	115.9	91.0
Other income (expense)	(7.2)	2.1	(27.2)	0.5
Income before taxes	139.7	57.4	399.0	342.0
Provision for income taxes	56.8	18.0	154.8	124.3
Net income from continuing operations	82.9	39.4	244.2	217.7
Discontinued operations, net of tax	1.1	(0.6)	0.9	4.7
Net income	84.0	38.8	245.1	222.4
Preferred stock dividends of subsidiary	(0.7)	(0.7)	(2.3)	(2.3)
Noncontrolling interest in subsidiaries	0	0	0.1	0.1
Net income attributed to common shareholders	$ 83.3	$ 38.1	$ 242.9	$ 220.2
Average shares of common stock
Basic (in shares)	80.2	79.8	80.2	79.3
Diluted (in shares)	81.1	80.2	80.6	79.9
Earnings per common share (basic)
Net income from continuing operations (in dollars per share)	$ 1.03	$ 0.49	$ 3.02	$ 2.72
Discontinued operations, net of tax (in dollars per share)	$ 0.01	$ (0.01)	$ 0.01	$ 0.06
Earnings per common share (basic) (in dollars per share)	$ 1.04	$ 0.48	$ 3.03	$ 2.78
Earnings per common share (diluted)
Net income from continuing operations (in dollars per share)	$ 1.01	$ 0.48	$ 3.00	$ 2.70
Discontinued operations, net of tax (in dollars per share)	$ 0.01	$ (0.01)	$ 0.01	$ 0.06
Earnings per common share (diluted) (in dollars per share)	$ 1.02	$ 0.47	$ 3.01	$ 2.76
Dividends per common share declared (in dollars per share)	$ 0.68	$ 0.68	$ 2.04	$ 2.04