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Revenue (Disaggregation of Revenues) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | $ 987.2 | $ 916.3
Electric [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 743.4 | 708.7
Electric [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 274.7 | 259.4
Electric [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 181.1 | 174.0
Electric [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 208.7 | 201.9
Electric [Member] | Wholesale [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 46.5 | 53.8
Electric [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 32.4 | 19.6
Gas [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 215.8 | 185.6
Gas [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 131.8 | 110.6
Gas [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 63.6 | 57.0
Gas [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 5.4 | 5.8
Gas [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 15.0 | 12.2
Other Utility [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 11.1 | 13.2
Other Utility [Member] | Steam [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 8.4 | 9.4
Other Utility [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 2.7 | 3.8
Non-utility and Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 16.9 | 8.8
Non-utility and Other [Member] | Transportation and other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 16.9 | 8.8
IPL [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 555.1 | 525.8
IPL [Member] | Electric [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 419.8 | 405.7
IPL [Member] | Electric [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 147.5 | 142.2
IPL [Member] | Electric [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 116.6 | 111.6
IPL [Member] | Electric [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 116.1 | 114.8
IPL [Member] | Electric [Member] | Wholesale [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 16.9 | 24.3
IPL [Member] | Electric [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 22.7 | 12.8
IPL [Member] | Gas [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 124.6 | 108.1
IPL [Member] | Gas [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 77.9 | 65.5
IPL [Member] | Gas [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 34.8 | 31.6
IPL [Member] | Gas [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 3.2 | 2.7
IPL [Member] | Gas [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 8.7 | 8.3
IPL [Member] | Other Utility [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 10.7 | 12.0
IPL [Member] | Other Utility [Member] | Steam [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 8.4 | 9.4
IPL [Member] | Other Utility [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 2.3 | 2.6
WPL [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 415.2 | 381.7
WPL [Member] | Electric [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 323.6 | 303.0
WPL [Member] | Electric [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 127.2 | 117.2
WPL [Member] | Electric [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 64.5 | 62.4
WPL [Member] | Electric [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 92.6 | 87.1
WPL [Member] | Electric [Member] | Wholesale [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 29.6 | 29.5
WPL [Member] | Electric [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 9.7 | 6.8
WPL [Member] | Gas [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 91.2 | 77.5
WPL [Member] | Gas [Member] | Retail - residential [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 53.9 | 45.1
WPL [Member] | Gas [Member] | Retail - commercial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 28.8 | 25.4
WPL [Member] | Gas [Member] | Retail - industrial [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 2.2 | 3.1
WPL [Member] | Gas [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 6.3 | 3.9
WPL [Member] | Other Utility [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | 0.4 | 1.2
WPL [Member] | Other Utility [Member] | Other [Member] |  |
Disaggregation of Revenue [Line Items] |  |
Revenue from contracts with customers | $ 0.4 | $ 1.2

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