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Segments Of Business (Schedule Of Segments Of Business) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Segment Reporting Information [Line Items] |  |  |  |
Revenues | $ 990.2 | $ 928.6 | $ 2,767.6 | $ 2,661.0
Operating income (loss) | 290.2 | 256.1 | 616.8 | 573.0
Net income attributable to common shareowners | 226.0 | 205.5 | 445.7 | 426.8
IPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 596.5 | 547.6 | 1,592.8 | 1,548.2
Operating income (loss) | 173.5 | 143.1 | 321.5 | 296.4
Net income attributable to common shareowners | 141.1 | 126.5 | 239.4 | 224.9
WPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 372.1 | 370.7 | 1,113.4 | 1,083.2
Operating income (loss) | 108.3 | 104.2 | 273.4 | 251.6
Net income attributable to common shareowners | 75.5 | 76.3 | 183.2 | 170.1
Electric [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 915.9 | 861.2 | 2,350.5 | 2,296.2
Operating income (loss) | 283.7 | 248.7 | 543.0 | 510.2
Electric [Member] | IPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 560.9 | 509.2 | 1,373.0 | 1,337.0
Operating income (loss) | 175.1 | 144.7 | 290.8 | 275.7
Electric [Member] | WPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 355.0 | 352.0 | 977.5 | 959.2
Operating income (loss) | 108.6 | 104.0 | 252.2 | 234.5
Gas [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 41.5 | 44.8 | 322.5 | 299.0
Operating income (loss) | (4.7) | (2.0) | 47.7 | 34.9
Gas [Member] | IPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 24.8 | 26.7 | 187.8 | 177.0
Operating income (loss) | (4.2) | (3.1) | 26.3 | 16.3
Gas [Member] | WPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 16.7 | 18.1 | 134.7 | 122.0
Operating income (loss) | (0.5) | 1.1 | 21.4 | 18.6
Other Utility [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 11.2 | 12.3 | 33.2 | 36.2
Operating income (loss) | 2.8 | 0.6 | 4.2 | 2.9
Other Utility [Member] | IPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 10.8 | 11.7 | 32.0 | 34.2
Operating income (loss) | 2.6 | 1.5 | 4.4 | 4.4
Other Utility [Member] | WPL [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 0.4 | 0.6 | 1.2 | 2.0
Operating income (loss) | 0.2 | (0.9) | (0.2) | (1.5)
Utility Business [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 968.6 | 918.3 | 2,706.2 | 2,631.4
Operating income (loss) | 281.8 | 247.3 | 594.9 | 548.0
Net income attributable to common shareowners | 216.6 | 202.8 | 422.6 | 395.0
Non-Utility [Member] | ATC Holdings, Non-Utility, Parent and Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Revenues | 21.6 | 10.3 | 61.4 | 29.6
Operating income (loss) | 8.4 | 8.8 | 21.9 | 25.0
Net income attributable to common shareowners | $ 9.4 | $ 2.7 | $ 23.1 | $ 31.8

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