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Revenues Revenue (Disaggregation of Revenues) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | $ 916.3 | $ 853.9
Electric [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 708.7 | 677.6
Electric [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 259.4 | 241.2
Electric [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 174.0 | 165.7
Electric [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 201.9 | 195.0
Electric [Member] | Wholesale [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 53.8 | 63.4
Electric [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 19.6 | 12.3
Gas [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 185.6 | 154.3
Gas [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 110.6 | 89.9
Gas [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 57.0 | 49.8
Gas [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 5.8 | 4.8
Gas [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 12.2 | 9.8
Other Utility [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 13.2 | 11.7
Other Utility [Member] | Steam [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 9.4 | 8.6
Other Utility [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 3.8 | 3.1
Non-utility and Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 8.8 | 10.3
Non-utility and Other [Member] | Transportation and other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 8.8 | 10.3
IPL [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 525.8 | 450.5
IPL [Member] | Electric [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 405.7 | 356.2
IPL [Member] | Electric [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 142.2 | 123.3
IPL [Member] | Electric [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 111.6 | 99.5
IPL [Member] | Electric [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 114.8 | 103.9
IPL [Member] | Electric [Member] | Wholesale [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 24.3 | 21.3
IPL [Member] | Electric [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 12.8 | 8.2
IPL [Member] | Gas [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 108.1 | 83.1
IPL [Member] | Gas [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 65.5 | 47.9
IPL [Member] | Gas [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 31.6 | 25.8
IPL [Member] | Gas [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 2.7 | 2.8
IPL [Member] | Gas [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 8.3 | 6.6
IPL [Member] | Other Utility [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 12.0 | 11.2
IPL [Member] | Other Utility [Member] | Steam [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 9.4 | 8.6
IPL [Member] | Other Utility [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 2.6 | 2.6
WPL [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 381.7 | 393.1
WPL [Member] | Electric [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 303.0 | 321.4
WPL [Member] | Electric [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 117.2 | 117.9
WPL [Member] | Electric [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 62.4 | 66.2
WPL [Member] | Electric [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 87.1 | 91.1
WPL [Member] | Electric [Member] | Wholesale [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 29.5 | 42.1
WPL [Member] | Electric [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 6.8 | 4.1
WPL [Member] | Gas [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 77.5 | 71.2
WPL [Member] | Gas [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 45.1 | 42.0
WPL [Member] | Gas [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 25.4 | 24.0
WPL [Member] | Gas [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 3.1 | 2.0
WPL [Member] | Gas [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 3.9 | 3.2
WPL [Member] | Other Utility [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 1.2 | 0.5
WPL [Member] | Other Utility [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | $ 1.2 | $ 0.5

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