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Revenues Revenue (Disaggregation of Revenues) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 928.6 $ 906.9 $ 2,661.0 $ 2,526.1
Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 861.2 840.6 2,296.2 2,199.1
Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 312.9 296.4 820.6 766.9
Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 215.4 205.9 561.7 537.3
Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 247.8 243.4 675.1 646.2
Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 50.0 63.5 146.9 186.3
Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 35.1 31.4 91.9 62.4
Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 44.8 45.8 299.0 262.7
Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 21.8 21.5 170.1 145.1
Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 11.0 12.9 87.3 81.4
Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 3.0 3.0 11.4 10.4
Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9.0 8.4 30.2 25.8
Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 12.3 11.2 36.2 34.4
Other Utility [Member] | Steam [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 8.7 8.3 26.5 25.3
Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 3.6 2.9 9.7 9.1
Non-utility and Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.3 9.3 29.6 29.9
Non-utility and Other [Member] | Transportation and other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.3 9.3 29.6 29.9
IPL [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 547.6 527.4 1,548.2 1,398.1
IPL [Member] | Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 509.2 489.0 1,337.0 1,217.6
IPL [Member] | Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 180.1 168.5 461.6 414.2
IPL [Member] | Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 146.0 137.5 371.8 340.6
IPL [Member] | Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 141.4 137.3 385.0 350.7
IPL [Member] | Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 18.2 26.7 56.4 71.3
IPL [Member] | Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 23.5 19.0 62.2 40.8
IPL [Member] | Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 26.7 27.4 177.0 147.2
IPL [Member] | Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 12.6 12.0 100.9 78.7
IPL [Member] | Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 6.4 7.8 49.8 44.4
IPL [Member] | Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 2.0 2.2 6.1 6.8
IPL [Member] | Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 5.7 5.4 20.2 17.3
IPL [Member] | Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 11.7 11.0 34.2 33.3
IPL [Member] | Other Utility [Member] | Steam [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 8.7 8.3 26.5 25.3
IPL [Member] | Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 3.0 2.7 7.7 8.0
WPL [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 370.7 370.2 1,083.2 1,098.1
WPL [Member] | Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 352.0 351.6 959.2 981.5
WPL [Member] | Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 132.8 127.9 359.0 352.7
WPL [Member] | Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 69.4 68.4 189.9 196.7
WPL [Member] | Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 106.4 106.1 290.1 295.5
WPL [Member] | Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 31.8 36.8 90.5 115.0
WPL [Member] | Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 11.6 12.4 29.7 21.6
WPL [Member] | Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 18.1 18.4 122.0 115.5
WPL [Member] | Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9.2 9.5 69.2 66.4
WPL [Member] | Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 4.6 5.1 37.5 37.0
WPL [Member] | Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 1.0 0.8 5.3 3.6
WPL [Member] | Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 3.3 3.0 10.0 8.5
WPL [Member] | Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 0.6 0.2 2.0 1.1
WPL [Member] | Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 0.6 $ 0.2 $ 2.0 $ 1.1