Selected Consolidated Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues	$ 880.1	$ 990.2	$ 790.2	$ 987.2	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 3,647.7	$ 3,534.5	$ 3,382.2
Operating income	160.9	290.2	149.8	176.8	121.4	256.1	151.2	165.7	777.7	694.4	671.2
Net income									567.4	522.3	467.5
Net income attributable to common shareowners	$ 111.5	$ 226.0	$ 94.6	$ 125.1	$ 85.3	$ 205.5	$ 100.4	$ 120.9	$ 557.2	$ 512.1	$ 457.3
Earnings per weighted average common share attributable to Alliant Energy common shareowners (in dollars per share)	$ 0.46	$ 0.94	$ 0.40	$ 0.53	$ 0.36	$ 0.87	$ 0.43	$ 0.52	$ 2.33	$ 2.19	$ 1.99
IPL [Member]
Revenues	$ 496.8	$ 596.5	$ 441.2	$ 555.1	$ 494.1	$ 547.6	$ 474.8	$ 525.8	$ 2,089.6	$ 2,042.3	$ 1,870.3
Operating income	81.3	173.5	74.5	73.5	54.4	143.1	77.7	75.6	402.8	350.8	304.1
Net income	47.2	143.7	47.5	55.9	41.6	129.1	54.2	49.3	294.3	274.2	227.0
Net income attributable to common shareowners	44.7	141.1	45.0	53.3	39.1	126.5	51.7	46.7	284.1	264.0	216.8
WPL [Member]
Revenues	362.3	372.1	326.1	415.2	369.4	370.7	330.8	381.7	1,475.7	1,452.6	1,472.8
Operating income	73.8	108.3	67.0	98.1	61.3	104.2	63.4	84.0	347.2	312.9	333.7
Net income	$ 49.8	$ 75.5	$ 42.0	$ 65.7	$ 38.0	$ 76.3	$ 39.8	$ 54.0	$ 233.0	$ 208.1	$ 186.6