Selected Consolidated Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 797.0	$ 924.6	$ 754.6	$ 843.8	$ 3,382.2	$ 3,320.0	$ 3,253.6
Operating income	129.7	231.5	149.3	142.9	99.9	162.6	128.6	145.9	653.4	537.0	577.0
Income from continuing operations, net of tax	93.8	168.8	94.3	99.0	63.0	128.8	84.4	97.6	455.9	373.8	380.7
Income (loss) from discontinued operations, net of tax	0.0	0.0	0.0	1.4	(0.3)	(0.4)	(0.5)	(1.1)	1.4	(2.3)	(2.5)
Net income									467.5	381.7	388.4
Net income attributable to common shareowners	$ 93.8	$ 168.8	$ 94.3	$ 100.4	$ 62.7	$ 128.4	$ 83.9	$ 96.5	457.3	371.5	378.2
Income from continuing operations, net of tax (in dollars per share)	$ 0.41	$ 0.73	$ 0.41	$ 0.43	$ 0.28	$ 0.57	$ 0.37	$ 0.43
Income from discontinued operations, net of tax (in dollars per share)	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00
Net income (in dollars per share)	$ 0.41	$ 0.73	$ 0.41	$ 0.44	$ 0.28	$ 0.57	$ 0.37	$ 0.43
IPL [Member]
Operating revenues	$ 472.2	$ 527.4	$ 420.2	$ 450.5	$ 434.5	$ 516.2	$ 411.0	$ 458.7	1,870.3	1,820.4	1,774.5
Operating income	49.2	131.8	66.3	49.6	34.9	125.9	48.0	62.0	296.9	270.8	241.9
Net income	18.9	123.0	45.3	39.8	26.5	116.7	34.4	48.2	227.0	225.8	196.2
Net income attributable to common shareowners	16.4	120.4	42.8	37.2	24.0	114.1	31.9	45.6	216.8	215.6	186.0
WPL [Member]
Operating revenues	374.7	370.2	334.8	393.1	352.2	397.0	334.3	375.6	1,472.8	1,459.1	1,435.1
Operating income	72.8	90.7	73.7	86.0	58.2	115.0	75.0	78.8	323.2	327.0	308.7
Net income	53.2	49.8	38.1	45.5	32.5	69.6	43.7	47.0	186.6	192.8	177.6
Net income attributable to common shareowners	$ 53.2	$ 49.8	$ 38.1	$ 45.5	$ 31.7	$ 69.0	$ 43.2	$ 46.5	$ 186.6	$ 190.4	$ 176.3