Segments Of Business (Schedule Of Segments Of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 916.3	$ 853.9
Operating income (loss)	165.7	147.2
Income from continuing operations, net of tax	120.9	99.0
Income from discontinued operations, net of tax	0.0	1.4
Net income attributable to common shareowners	120.9	100.4
IPL [Member]
Segment Reporting Information [Line Items]
Revenues	525.8	450.5
Operating income (loss)	75.6	51.3
Net income attributable to common shareowners	46.7	37.2
WPL [Member]
Segment Reporting Information [Line Items]
Revenues	381.7	393.1
Operating income (loss)	84.0	88.6
Net income attributable to common shareowners	54.0	45.5
Electric [Member]
Segment Reporting Information [Line Items]
Revenues	708.7	677.6
Operating income (loss)	126.7	110.9
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	405.7	356.2
Operating income (loss)	57.5	35.3
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	303.0	321.4
Operating income (loss)	69.2	75.6
Gas [Member]
Segment Reporting Information [Line Items]
Revenues	185.6	154.3
Operating income (loss)	31.5	28.6
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	108.1	83.1
Operating income (loss)	16.9	14.6
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	77.5	71.2
Operating income (loss)	14.6	14.0
Other Utility [Member]
Segment Reporting Information [Line Items]
Revenues	13.2	11.7
Operating income (loss)	1.4	0.4
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	12.0	11.2
Operating income (loss)	1.2	1.4
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	1.2	0.5
Operating income (loss)	0.2	(1.0)
Utility Business [Member]
Segment Reporting Information [Line Items]
Revenues	907.5	843.6
Operating income (loss)	159.6	139.9
Income from continuing operations, net of tax		82.7
Income from discontinued operations, net of tax		0.0
Net income attributable to common shareowners	100.7	82.7
Non-Utility [Member] | ATC Holdings, Non-utility, Parent and Other [Member]
Segment Reporting Information [Line Items]
Revenues	8.8	10.3
Operating income (loss)	6.1	7.3
Income from continuing operations, net of tax		16.3
Income from discontinued operations, net of tax		1.4
Net income attributable to common shareowners	$ 20.2	$ 17.7