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Benefit Plans (Expected Benefit Payments) (Details)
$ in Millions
Dec. 31, 2019
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 $ 104.1
Expected benefit payments, 2021 92.2
Expected benefit payments, 2022 94.1
Expected benefit payments, 2023 94.6
Expected benefit payments, 2024 95.0
Expected benefit payments, 2025 - 2029 466.8
Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 86.4
Expected benefit payments, 2021 74.5
Expected benefit payments, 2022 76.6
Expected benefit payments, 2023 77.5
Expected benefit payments, 2024 78.2
Expected benefit payments, 2025 - 2029 390.2
Other Postretirement Benefits Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 17.7
Expected benefit payments, 2021 17.7
Expected benefit payments, 2022 17.5
Expected benefit payments, 2023 17.1
Expected benefit payments, 2024 16.8
Expected benefit payments, 2025 - 2029 76.6
IPL [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 42.3
Expected benefit payments, 2021 42.4
Expected benefit payments, 2022 43.7
Expected benefit payments, 2023 44.7
Expected benefit payments, 2024 43.5
Expected benefit payments, 2025 - 2029 213.1
IPL [Member] | Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 35.1
Expected benefit payments, 2021 35.3
Expected benefit payments, 2022 36.6
Expected benefit payments, 2023 37.7
Expected benefit payments, 2024 36.6
Expected benefit payments, 2025 - 2029 182.5
IPL [Member] | Other Postretirement Benefits Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 7.2
Expected benefit payments, 2021 7.1
Expected benefit payments, 2022 7.1
Expected benefit payments, 2023 7.0
Expected benefit payments, 2024 6.9
Expected benefit payments, 2025 - 2029 30.6
WPL [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 39.3
Expected benefit payments, 2021 39.4
Expected benefit payments, 2022 39.2
Expected benefit payments, 2023 39.2
Expected benefit payments, 2024 39.8
Expected benefit payments, 2025 - 2029 195.4
WPL [Member] | Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 32.0
Expected benefit payments, 2021 32.0
Expected benefit payments, 2022 32.1
Expected benefit payments, 2023 32.4
Expected benefit payments, 2024 33.1
Expected benefit payments, 2025 - 2029 165.2
WPL [Member] | Other Postretirement Benefits Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected benefit payments, 2020 7.3
Expected benefit payments, 2021 7.4
Expected benefit payments, 2022 7.1
Expected benefit payments, 2023 6.8
Expected benefit payments, 2024 6.7
Expected benefit payments, 2025 - 2029 $ 30.2