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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

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