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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues	$ 804.1	$ 843.1	$ 750.3	$ 952.8	$ 832.6	$ 866.6	$ 718.0	$ 859.6	$ 3,350.3	$ 3,276.8	$ 3,094.5
Operating income	91.3	194.8	103.3	154.2	108.6	201.4	103.2	120.7	543.6	533.9	519.7
Income from continuing operations, net of tax	60.2	155.2	62.1	108.0	66.5	158.9	65.9	72.9	385.5	364.2	324.9
Loss from discontinued operations, net of tax	(0.2)	(1.9)	(0.3)	0	(1.0)	(1.3)	(0.6)	(3.0)	(2.4)	(5.9)	(5.1)
Net income									393.3	376.2	335.7
Net income (loss) attributable to common shareowners	60.0	153.3	61.8	108.0	65.5	157.6	65.3	69.9	383.1	358.3	319.8
Income from continuing operations, net of tax (in dollars per share)	$ 0.54	$ 1.40	$ 0.56	$ 0.97	$ 0.60	$ 1.43	$ 0.59	$ 0.66
Loss from discontinued operations, net of tax (in dollars per share)	$ 0.00	$ (0.02)	$ 0.00	$ 0.00	$ (0.01)	$ (0.01)	$ 0.00	$ (0.03)
Net income (in dollars per share)	$ 0.54	$ 1.38	$ 0.56	$ 0.97	$ 0.59	$ 1.42	$ 0.59	$ 0.63
IPL [Member]
Operating revenues	431.1	476.2	411.9	528.9	463.1	494.4	383.4	477.9	1,848.1	1,818.8	1,650.3
Operating income	23.8	93.9	34.0	57.5	36.2	100.0	34.7	41.1	209.2	212.0	200.3
Net income	22.6	105.1	20.9	46.0	21.1	112.6	24.7	31.5	194.6	189.9	150.2
Net income (loss) attributable to common shareowners	20.1	102.5	18.4	43.4	18.5	110.0	22.2	22.9	184.4	173.6	137.6
WPL [Member]
Operating revenues	359.8	354.4	324.5	410.4	355.7	360.9	319.9	369.8	1,449.1	1,406.3	1,392.0
Operating income	59.4	93.9	60.0	87.7	67.8	95.9	59.7	72.7	301.0	296.1	284.8
Net income	29.8	61.6	34.6	54.8	38.2	61.3	34.4	43.6	180.8	177.5	165.7
Net income (loss) attributable to common shareowners	$ 29.1	$ 61.6	$ 34.6	$ 54.8	$ 38.2	$ 61.3	$ 34.4	$ 42.0	$ 180.1	$ 175.9	$ 162.4