Selected Consolidated Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 3,534.5	$ 3,382.2	$ 3,320.0
Operating income	121.4	256.1	151.2	165.7	134.0	236.3	153.7	147.2	694.4	671.2	554.1
Income from continuing operations, net of tax	85.3	205.5	100.4	120.9	93.8	168.8	94.3	99.0	512.1	455.9	373.8
Income (loss) from discontinued operations, net of tax	0.0	0.0	0.0	0.0	0.0	0.0	0.0	1.4	0.0	1.4	(2.3)
Net income									522.3	467.5	381.7
Net income attributable to common shareowners	$ 85.3	$ 205.5	$ 100.4	$ 120.9	$ 93.8	$ 168.8	$ 94.3	$ 100.4	512.1	457.3	371.5
Income from continuing operations, net of tax (in dollars per share)	$ 0.36	$ 0.87	$ 0.43	$ 0.52	$ 0.41	$ 0.73	$ 0.41	$ 0.43
Income from discontinued operations, net of tax (in dollars per share)	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01
Net income (in dollars per share)	$ 0.36	$ 0.87	$ 0.43	$ 0.52	$ 0.41	$ 0.73	$ 0.41	$ 0.44
IPL [Member]
Revenues	$ 494.1	$ 547.6	$ 474.8	$ 525.8	$ 472.2	$ 527.4	$ 420.2	$ 450.5	2,042.3	1,870.3	1,820.4
Operating income	54.4	143.1	77.7	75.6	50.9	133.8	68.1	51.3	350.8	304.1	277.6
Net income	41.6	129.1	54.2	49.3	18.9	123.0	45.3	39.8	274.2	227.0	225.8
Net income attributable to common shareowners	39.1	126.5	51.7	46.7	16.4	120.4	42.8	37.2	264.0	216.8	215.6
WPL [Member]
Revenues	369.4	370.7	330.8	381.7	374.7	370.2	334.8	393.1	1,452.6	1,472.8	1,459.1
Operating income	61.3	104.2	63.4	84.0	75.4	93.5	76.2	88.6	312.9	333.7	337.2
Net income	38.0	76.3	39.8	54.0	53.2	49.8	38.1	45.5	208.1	186.6	192.8
Net income attributable to common shareowners	$ 38.0	$ 76.3	$ 39.8	$ 54.0	$ 53.2	$ 49.8	$ 38.1	$ 45.5	$ 208.1	$ 186.6	$ 190.4