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Revenue (Disaggregation of Revenues) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 790.2 $ 816.1 $ 1,777.4 $ 1,732.4
Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 691.2 726.3 1,434.6 1,435.0
Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 233.3 248.3 508.0 507.7
Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 171.3 172.3 352.4 346.3
Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 214.2 225.4 422.9 427.3
Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 41.0 43.1 87.5 96.9
Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 31.4 37.2 63.8 56.8
Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 65.2 68.6 281.0 254.2
Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 35.2 37.7 167.0 148.3
Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 16.1 19.3 79.7 76.3
Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 2.0 2.6 7.4 8.4
Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 11.9 9.0 26.9 21.2
Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.9 10.7 22.0 23.9
Other Utility [Member] | Steam [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9.2 8.4 17.6 17.8
Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 1.7 2.3 4.4 6.1
Non-utility and Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 22.9 10.5 39.8 19.3
Non-utility and Other [Member] | Transportation and other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 22.9 10.5 39.8 19.3
IPL [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 441.2 474.8 996.3 1,000.6
IPL [Member] | Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 392.3 422.1 812.1 827.8
IPL [Member] | Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 125.9 139.3 273.4 281.5
IPL [Member] | Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 111.4 114.2 228.0 225.8
IPL [Member] | Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 118.4 128.8 234.5 243.6
IPL [Member] | Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 14.6 13.9 31.5 38.2
IPL [Member] | Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 22.0 25.9 44.7 38.7
IPL [Member] | Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 38.4 42.2 163.0 150.3
IPL [Member] | Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 20.0 22.8 97.9 88.3
IPL [Member] | Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 8.8 11.8 43.6 43.4
IPL [Member] | Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 1.6 1.4 4.8 4.1
IPL [Member] | Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 8.0 6.2 16.7 14.5
IPL [Member] | Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 10.5 10.5 21.2 22.5
IPL [Member] | Other Utility [Member] | Steam [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9.2 8.4 17.6 17.8
IPL [Member] | Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 1.3 2.1 3.6 4.7
WPL [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 326.1 330.8 741.3 712.5
WPL [Member] | Electric [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 298.9 304.2 622.5 607.2
WPL [Member] | Electric [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 107.4 109.0 234.6 226.2
WPL [Member] | Electric [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 59.9 58.1 124.4 120.5
WPL [Member] | Electric [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 95.8 96.6 188.4 183.7
WPL [Member] | Electric [Member] | Wholesale [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 26.4 29.2 56.0 58.7
WPL [Member] | Electric [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 9.4 11.3 19.1 18.1
WPL [Member] | Gas [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 26.8 26.4 118.0 103.9
WPL [Member] | Gas [Member] | Retail - residential [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 15.2 14.9 69.1 60.0
WPL [Member] | Gas [Member] | Retail - commercial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 7.3 7.5 36.1 32.9
WPL [Member] | Gas [Member] | Retail - industrial [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 0.4 1.2 2.6 4.3
WPL [Member] | Gas [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 3.9 2.8 10.2 6.7
WPL [Member] | Other Utility [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers 0.4 0.2 0.8 1.4
WPL [Member] | Other Utility [Member] | Other [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from contracts with customers $ 0.4 $ 0.2 $ 0.8 $ 1.4