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Benefit Plans (Expected Benefit Payments) (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 $ 91.0
Expected benefit payments, 2020 104.2
Expected benefit payments, 2021 92.5
Expected benefit payments, 2022 93.8
Expected benefit payments, 2023 94.8
Expected benefit payments, 2024 - 2028 466.0
Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 71.7
Expected benefit payments, 2020 86.1
Expected benefit payments, 2021 74.7
Expected benefit payments, 2022 76.3
Expected benefit payments, 2023 77.8
Expected benefit payments, 2024 - 2028 388.3
Other Postretirement Benefits Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 19.3
Expected benefit payments, 2020 18.1
Expected benefit payments, 2021 17.8
Expected benefit payments, 2022 17.5
Expected benefit payments, 2023 17.0
Expected benefit payments, 2024 - 2028 77.7
IPL [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 40.3
Expected benefit payments, 2020 42.4
Expected benefit payments, 2021 42.3
Expected benefit payments, 2022 43.5
Expected benefit payments, 2023 44.4
Expected benefit payments, 2024 - 2028 214.2
IPL [Member] | Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 33.0
Expected benefit payments, 2020 35.1
Expected benefit payments, 2021 35.1
Expected benefit payments, 2022 36.4
Expected benefit payments, 2023 37.5
Expected benefit payments, 2024 - 2028 182.9
IPL [Member] | Other Postretirement Benefits Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 7.3
Expected benefit payments, 2020 7.3
Expected benefit payments, 2021 7.2
Expected benefit payments, 2022 7.1
Expected benefit payments, 2023 6.9
Expected benefit payments, 2024 - 2028 31.3
WPL [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 40.2
Expected benefit payments, 2020 39.4
Expected benefit payments, 2021 39.3
Expected benefit payments, 2022 39.1
Expected benefit payments, 2023 39.3
Expected benefit payments, 2024 - 2028 196.0
WPL [Member] | Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 31.4
Expected benefit payments, 2020 31.8
Expected benefit payments, 2021 31.9
Expected benefit payments, 2022 32.0
Expected benefit payments, 2023 32.5
Expected benefit payments, 2024 - 2028 165.3
WPL [Member] | Other Postretirement Benefits Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2019 8.8
Expected benefit payments, 2020 7.6
Expected benefit payments, 2021 7.4
Expected benefit payments, 2022 7.1
Expected benefit payments, 2023 6.8
Expected benefit payments, 2024 - 2028 $ 30.7