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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,844.7 | $ 1,737.3
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 817.6 | 820.6
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 524.0 | 542.3
Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 134.0 | 111.6
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 111.8 | 110.8
IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 108.2 | 0.0
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.5 | 28.0
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 21.1 | 23.6
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 88.5 | 100.4
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,399.3 | 1,279.0
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 776.8 | 783.1
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 262.5 | 274.0
IPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 87.9 | 55.4
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 76.2 | 76.3
IPL [Member] | IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 108.2 | 0.0
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 18.3 | 15.1
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 21.1 | 23.6
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 48.3 | 51.5
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 445.4 | 458.3
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 40.8 | 37.5
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 261.5 | 268.3
WPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 46.1 | 56.2
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 35.6 | 34.5
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 21.2 | 12.9
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 40.2 | $ 48.9

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