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Revenues Revenue (Disaggregation of Revenues) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 816.1 $ 765.3 $ 1,732.4 $ 1,619.2
Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 726.3 680.9 1,435.0 1,358.5
Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 248.3 229.3 507.7 470.5
Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 172.3 165.7 346.3 331.4
Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 225.4 207.8 427.3 402.8
Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 43.1 59.4 96.9 122.8
Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 37.2 18.7 56.8 31.0
Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 68.6 62.6 254.2 216.9
Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 37.7 33.7 148.3 123.6
Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 19.3 18.7 76.3 68.5
Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 2.6 2.6 8.4 7.4
Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9.0 7.6 21.2 17.4
Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.7 11.5 23.9 23.2
Other Utility [Member] | Steam [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 8.4 8.4 17.8 17.0
Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 2.3 3.1 6.1 6.2
Non-utility and Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.5 10.3 19.3 20.6
Non-utility and Other [Member] | Transportation and other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.5 10.3 19.3 20.6
IPL [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 474.8 420.2 1,000.6 870.7
IPL [Member] | Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 422.1 372.4 827.8 728.6
IPL [Member] | Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 139.3 122.4 281.5 245.7
IPL [Member] | Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 114.2 103.6 225.8 203.1
IPL [Member] | Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 128.8 109.5 243.6 213.4
IPL [Member] | Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 13.9 23.3 38.2 44.6
IPL [Member] | Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 25.9 13.6 38.7 21.8
IPL [Member] | Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 42.2 36.7 150.3 119.8
IPL [Member] | Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 22.8 18.8 88.3 66.7
IPL [Member] | Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 11.8 10.8 43.4 36.6
IPL [Member] | Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 1.4 1.8 4.1 4.6
IPL [Member] | Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 6.2 5.3 14.5 11.9
IPL [Member] | Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.5 11.1 22.5 22.3
IPL [Member] | Other Utility [Member] | Steam [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 8.4 8.4 17.8 17.0
IPL [Member] | Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 2.1 2.7 4.7 5.3
WPL [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 330.8 334.8 712.5 727.9
WPL [Member] | Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 304.2 308.5 607.2 629.9
WPL [Member] | Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 109.0 106.9 226.2 224.8
WPL [Member] | Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 58.1 62.1 120.5 128.3
WPL [Member] | Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 96.6 98.3 183.7 189.4
WPL [Member] | Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 29.2 36.1 58.7 78.2
WPL [Member] | Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 11.3 5.1 18.1 9.2
WPL [Member] | Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 26.4 25.9 103.9 97.1
WPL [Member] | Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 14.9 14.9 60.0 56.9
WPL [Member] | Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 7.5 7.9 32.9 31.9
WPL [Member] | Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 1.2 0.8 4.3 2.8
WPL [Member] | Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 2.8 2.3 6.7 5.5
WPL [Member] | Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 0.2 0.4 1.4 0.9
WPL [Member] | Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 0.2 $ 0.4 $ 1.4 $ 0.9