QUARTERLY FINANCIAL INFORMATION (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
Quarterly financial information
Operating revenues	$ 4,144.2	$ 3,485.5	$ 3,012.9
Operating income (loss)	507.0	439.5	379.5
Net income (loss) from continuing operations	278.1	267.5	238.9
Net income (loss)	279.9	354.8	284.3
Net income (loss) attributed to common shareholders	276.9	351.8	281.4
Earnings (loss) per common share (basic)
Net income (loss) from continuing operations (in dollars per share)	$ 3.43	$ 3.33	$ 3.00
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.10	$ 0.58
Earnings per common share (basic) (in dollars per share)	$ 3.45	$ 4.43	$ 3.58
Earnings (loss) per common share (diluted)
Net income (loss) from continuing operations (in dollars per share)	$ 3.41	$ 3.30	$ 2.98
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.09	$ 0.57
Earnings per common share (diluted) (in dollars per share)	$ 3.43	$ 4.39	$ 3.55
Amounts reflecting IES's retail energy business in discontinued operations
Quarterly financial information
Operating revenues	4,144.2	3,485.5		1,012.3	657.1	836.8	1,638.0	1,018.8	622.2	708.1	1,136.4
Operating income (loss)	507.0	439.5		116.9	134.6	23.2	232.3	130.1	41.5	56.1	211.8
Net income (loss) from continuing operations	278.1	267.5		53.8	75.3	8.8	140.2	74.2	26.9	36.8	129.6
Net income (loss)	279.9	354.8		34.8	84.0	8.0	153.1	132.4	38.8	(4.7)	188.3
Net income (loss) attributed to common shareholders	276.9	351.8		34.0	83.3	7.2	152.4	131.6	38.1	(5.4)	187.5
Earnings (loss) per common share (basic)
Net income (loss) from continuing operations (in dollars per share)	$ 3.43	$ 3.33		$ 0.66	$ 0.93	$ 0.10	$ 1.74	$ 0.91	$ 0.33	$ 0.45	$ 1.64
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.10		$ (0.24)	$ 0.11	$ (0.01)	$ 0.16	$ 0.73	$ 0.15	$ (0.52)	$ 0.74
Earnings per common share (basic) (in dollars per share)	$ 3.45	$ 4.43		$ 0.42	$ 1.04	$ 0.09	$ 1.90	$ 1.64	$ 0.48	$ (0.07)	$ 2.38
Earnings (loss) per common share (diluted)
Net income (loss) from continuing operations (in dollars per share)	$ 3.41	$ 3.30		$ 0.66	$ 0.91	$ 0.10	$ 1.73	$ 0.91	$ 0.32	$ 0.45	$ 1.63
Discontinued operations, net of tax (in dollars per share)	$ 0.02	$ 1.09		$ (0.24)	$ 0.11	$ (0.01)	$ 0.16	$ 0.72	$ 0.15	$ (0.52)	$ 0.74
Earnings per common share (diluted) (in dollars per share)	$ 3.43	$ 4.39		$ 0.42	$ 1.02	$ 0.09	$ 1.89	$ 1.63	$ 0.47	$ (0.07)	$ 2.37
Previously reported amounts reflecting IES's retail energy business in continuing operations
Quarterly financial information
Operating revenues		5,634.6			1,187.9	1,432.6	2,924.9	1,710.7	1,129.7	1,116.0	1,678.2
Operating income (loss)		567.7			146.9	26.1	253.2	226.2	55.3	(6.9)	293.1
Net income (loss) from continuing operations		350.0			82.9	8.1	153.2	132.3	39.4	(3.9)	182.2
Net income (loss)		354.8			84.0	8.0	153.1	132.4	38.8	(4.7)	188.3
Net income (loss) attributed to common shareholders		$ 351.8			$ 83.3	$ 7.2	$ 152.4	$ 131.6	$ 38.1	$ (5.4)	$ 187.5
Earnings (loss) per common share (basic)
Net income (loss) from continuing operations (in dollars per share)		$ 4.37			$ 1.03	$ 0.09	$ 1.90	$ 1.64	$ 0.49	$ (0.06)	$ 2.30
Discontinued operations, net of tax (in dollars per share)		$ 0.06			$ 0.01	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.01)	$ 0.08
Earnings per common share (basic) (in dollars per share)		$ 4.43			$ 1.04	$ 0.09	$ 1.90	$ 1.64	$ 0.48	$ (0.07)	$ 2.38
Earnings (loss) per common share (diluted)
Net income (loss) from continuing operations (in dollars per share)		$ 4.33			$ 1.01	$ 0.09	$ 1.89	$ 1.63	$ 0.48	$ (0.06)	$ 2.29
Discontinued operations, net of tax (in dollars per share)		$ 0.06			$ 0.01	$ 0.00	$ 0.00	$ 0.00	$ (0.01)	$ (0.01)	$ 0.08
Earnings per common share (diluted) (in dollars per share)		$ 4.39			$ 1.02	$ 0.09	$ 1.89	$ 1.63	$ 0.47	$ (0.07)	$ 2.37