Selected Consolidated Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues	$ 750.9	$ 887.6	$ 690.3	$ 765.7	$ 760.8	$ 870.9	$ 712.5	$ 877.2	$ 3,094.5	$ 3,221.4	$ 3,262.1
Operating income	101.6	213.7	108.8	95.6	106.0	209.4	70.6	127.3	519.7	513.3	560.0
Income from continuing operations, net of tax	71.1	149.0	65.5	39.3	63.9	136.9	50.3	72.0	324.9	323.1	291.5
Income (loss) from discontinued operations, net of tax	(2.8)	1.7	0.4	(4.4)	(6.9)	(14.9)	0.8	1.5	(5.1)	(19.5)	(3.9)
Net income (loss)									335.7	321.9	306.3
Net income (loss) attributable to common shareowners	68.3	150.7	65.9	34.9	57.0	122.0	51.1	73.5	319.8	303.6	287.6
Income from continuing operations, net of tax (in dollars per share)	$ 0.64	$ 1.34	$ 0.60	$ 0.36	$ 0.58	$ 1.23	$ 0.45	$ 0.65
Income (loss) from discontinued operations, net of tax (in dollars per share)	$ (0.02)	$ 0.02	$ 0.00	$ (0.04)	$ (0.07)	$ (0.13)	$ 0.01	$ 0.01
Net income (in dollars per share)	$ 0.62	$ 1.36	$ 0.60	$ 0.32	$ 0.51	$ 1.10	$ 0.46	$ 0.66
IPL [Member]
Operating revenues	393.2	497.7	360.7	398.7	404.0	484.5	374.1	477.5	1,650.3	1,740.1	1,795.8
Operating income	29.0	112.0	35.4	23.9	26.3	116.5	18.5	47.1	200.3	208.4	261.9
Net income (loss)	25.6	106.5	19.6	(1.5)	9.8	101.4	1.0	27.1	150.2	139.3	143.4
Net income (loss) attributable to common shareowners	22.4	103.3	16.6	(4.7)	6.6	98.3	(2.3)	21.7	137.6	124.3	128.0
WPL [Member]
Operating revenues	345.2	376.6	315.7	354.5	344.5	374.6	326.8	388.5	1,392.0	1,434.4	1,423.6
Operating income	65.3	93.4	61.5	64.6	73.2	86.9	45.5	74.8	284.8	280.4	278.8
Net income (loss)	41.0	56.7	36.1	31.9	42.7	51.4	25.0	44.4	165.7	163.5	152.3
Net income (loss) attributable to common shareowners	$ 40.2	$ 55.9	$ 35.2	$ 31.1	$ 41.9	$ 50.6	$ 24.1	$ 43.6	$ 162.4	$ 160.2	$ 149.0