Selected Consolidated Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues	$ 740.1	$ 898.9	$ 717.2	$ 897.4	$ 804.1	$ 843.1	$ 750.3	$ 952.8	$ 3,253.6	$ 3,350.3	$ 3,276.8
Operating income	79.2	235.9	109.0	152.9	91.3	194.8	103.3	154.2	577.0	543.6	533.9
Income from continuing operations, net of tax	35.2	180.0	68.9	96.6	60.2	155.2	62.1	108.0	380.7	385.5	364.2
Loss from discontinued operations, net of tax	(1.1)	(0.1)	(1.3)	0.0	(0.2)	(1.9)	(0.3)	0.0	(2.5)	(2.4)	(5.9)
Net income									388.4	393.3	376.2
Net income (loss) attributable to common shareowners	$ 34.1	$ 179.9	$ 67.6	$ 96.6	$ 60.0	$ 153.3	$ 61.8	$ 108.0	378.2	383.1	358.3
Income from continuing operations, net of tax (in dollars per share)	$ 0.31	$ 1.59	$ 0.61	$ 0.87	$ 0.54	$ 1.40	$ 0.56	$ 0.97
Loss from discontinued operations, net of tax (in dollars per share)	(0.01)	0.00	(0.01)	0.00	0.00	(0.02)	0.00	0.00
Net income (in dollars per share)	$ 0.30	$ 1.59	$ 0.60	$ 0.87	$ 0.54	$ 1.38	$ 0.56	$ 0.97
IPL [Member]
Operating revenues	$ 398.7	$ 504.6	$ 382.2	$ 489.0	$ 431.1	$ 476.2	$ 411.9	$ 528.9	1,774.5	1,848.1	1,818.8
Operating income	25.6	117.0	33.8	65.5	23.8	93.9	34.0	57.5	241.9	209.2	212.0
Net income	8.0	119.1	19.0	50.1	21.4	104.6	20.5	45.3	196.2	191.8	188.3
Net income (loss) attributable to common shareowners	5.5	116.5	16.5	47.5	18.9	102.0	18.0	42.7	186.0	181.6	172.0
WPL [Member]
Operating revenues	330.7	382.6	324.7	397.1	359.8	354.4	324.5	410.4	1,435.1	1,449.1	1,406.3
Operating income	49.3	110.3	68.3	80.8	59.4	93.9	60.0	87.7	308.7	301.0	296.1
Net income	24.4	68.4	39.7	45.1	33.3	57.9	34.7	55.2	177.6	181.1	179.1
Net income (loss) attributable to common shareowners	$ 24.2	$ 68.0	$ 39.2	$ 44.9	$ 32.6	$ 57.9	$ 34.7	$ 55.2	$ 176.3	$ 180.4	$ 177.5