Segments Of Business (Schedule Of Segments Of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 816.1	$ 765.3	$ 1,732.4	$ 1,619.2
Operating income (loss)	151.2	153.7	316.9	300.9
Income from continuing operations, net of tax	100.4	94.3	221.3	193.3
Income from discontinued operations, net of tax	0.0	0.0	0.0	1.4
Net income attributable to common shareowners	100.4	94.3	221.3	194.7
IPL [Member]
Segment Reporting Information [Line Items]
Revenues	474.8	420.2	1,000.6	870.7
Operating income (loss)	77.7	68.1	153.3	119.4
Net income attributable to common shareowners	51.7	42.8	98.4	80.0
WPL [Member]
Segment Reporting Information [Line Items]
Revenues	330.8	334.8	712.5	727.9
Operating income (loss)	63.4	76.2	147.4	164.8
Net income attributable to common shareowners	39.8	38.1	93.8	83.6
Electric [Member]
Segment Reporting Information [Line Items]
Revenues	726.3	680.9	1,435.0	1,358.5
Operating income (loss)	134.8	139.3	261.5	250.2
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	422.1	372.4	827.8	728.6
Operating income (loss)	73.5	63.9	131.0	99.2
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	304.2	308.5	607.2	629.9
Operating income (loss)	61.3	75.4	130.5	151.0
Gas [Member]
Segment Reporting Information [Line Items]
Revenues	68.6	62.6	254.2	216.9
Operating income (loss)	5.4	4.4	36.9	33.0
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	42.2	36.7	150.3	119.8
Operating income (loss)	2.5	2.6	19.4	17.2
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	26.4	25.9	103.9	97.1
Operating income (loss)	2.9	1.8	17.5	15.8
Other Utility [Member]
Segment Reporting Information [Line Items]
Revenues	10.7	11.5	23.9	23.2
Operating income (loss)	0.9	0.6	2.3	1.0
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	10.5	11.1	22.5	22.3
Operating income (loss)	1.7	1.6	2.9	3.0
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	0.2	0.4	1.4	0.9
Operating income (loss)	(0.8)	(1.0)	(0.6)	(2.0)
Utility Business [Member]
Segment Reporting Information [Line Items]
Revenues	805.6	755.0	1,713.1	1,598.6
Operating income (loss)	141.1	144.3	300.7	284.2
Income from continuing operations, net of tax				163.6
Income from discontinued operations, net of tax				0.0
Net income attributable to common shareowners	91.5	80.9	192.2	163.6
Non-Utility [Member] | ATC Holdings, Non-Utility, Parent and Other [Member]
Segment Reporting Information [Line Items]
Revenues	10.5	10.3	19.3	20.6
Operating income (loss)	10.1	9.4	16.2	16.7
Income from continuing operations, net of tax				29.7
Income from discontinued operations, net of tax				1.4
Net income attributable to common shareowners	$ 8.9	$ 13.4	$ 29.1	$ 31.1