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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,792.4 | $ 1,737.3
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 802.9 | 820.6
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 532.6 | 542.3
Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 112.1 | 110.8
EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 107.3 | 111.6
IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 106.5 | 0.0
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 23.0 | 23.6
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 16.9 | 28.0
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 91.1 | 100.4
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,351.4 | 1,279.0
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 770.2 | 783.1
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 269.3 | 274.0
IPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 77.5 | 76.3
IPL [Member] | EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 53.6 | 55.4
IPL [Member] | IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 106.5 | 0.0
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 23.0 | 23.6
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 8.0 | 15.1
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 43.3 | 51.5
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 441.0 | 458.3
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 32.7 | 37.5
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 263.3 | 268.3
WPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 34.6 | 34.5
WPL [Member] | EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 53.7 | 56.2
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 8.9 | 12.9
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 47.8 | $ 48.9

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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