Segments Of Business (Schedule Of Segments Of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues	$ 928.6	$ 906.9	$ 2,661.0	$ 2,526.1
Operating income (loss)	256.1	236.3	573.0	537.2
Income from continuing operations, net of tax	205.5	168.8	426.8	362.1
Income from discontinued operations, net of tax	0.0	0.0	0.0	1.4
Net income attributable to common shareowners	205.5	168.8	426.8	363.5
IPL [Member]
Segment Reporting Information [Line Items]
Revenues	547.6	527.4	1,548.2	1,398.1
Operating income (loss)	143.1	133.8	296.4	253.2
Net income attributable to common shareowners	126.5	120.4	224.9	200.4
WPL [Member]
Segment Reporting Information [Line Items]
Revenues	370.7	370.2	1,083.2	1,098.1
Operating income (loss)	104.2	93.5	251.6	258.3
Net income attributable to common shareowners	76.3	49.8	170.1	133.4
Electric [Member]
Segment Reporting Information [Line Items]
Revenues	861.2	840.6	2,296.2	2,199.1
Operating income (loss)	248.7	236.7	510.2	486.9
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	509.2	489.0	1,337.0	1,217.6
Operating income (loss)	144.7	140.0	275.7	239.2
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	352.0	351.6	959.2	981.5
Operating income (loss)	104.0	96.7	234.5	247.7
Gas [Member]
Segment Reporting Information [Line Items]
Revenues	44.8	45.8	299.0	262.7
Operating income (loss)	(2.0)	(1.7)	34.9	31.3
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	26.7	27.4	177.0	147.2
Operating income (loss)	(3.1)	(1.8)	16.3	15.4
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	18.1	18.4	122.0	115.5
Operating income (loss)	1.1	0.1	18.6	15.9
Other Utility [Member]
Segment Reporting Information [Line Items]
Revenues	12.3	11.2	36.2	34.4
Operating income (loss)	0.6	(7.7)	2.9	(6.7)
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Revenues	11.7	11.0	34.2	33.3
Operating income (loss)	1.5	(4.4)	4.4	(1.4)
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Revenues	0.6	0.2	2.0	1.1
Operating income (loss)	(0.9)	(3.3)	(1.5)	(5.3)
Utility Business [Member]
Segment Reporting Information [Line Items]
Revenues	918.3	897.6	2,631.4	2,496.2
Operating income (loss)	247.3	227.3	548.0	511.5
Income from continuing operations, net of tax				333.8
Income from discontinued operations, net of tax				0.0
Net income attributable to common shareowners	202.8	170.2	395.0	333.8
Non-Utility [Member] | ATC Holdings, Non-Utility, Parent and Other [Member]
Segment Reporting Information [Line Items]
Revenues	10.3	9.3	29.6	29.9
Operating income (loss)	8.8	9.0	25.0	25.7
Income from continuing operations, net of tax				28.3
Income from discontinued operations, net of tax				1.4
Net income attributable to common shareowners	$ 2.7	$ (1.4)	$ 31.8	$ 29.7