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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,694.7 | $ 1,666.7
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 798.3 | 778.2
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 519.8 | 548.0
Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 111.6 | 109.3
EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 110.4 | 63.8
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.3 | 45.3
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 24.3 | 25.5
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 104.0 | 96.6
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,249.9 | 1,231.6
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 762.4 | 750.5
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 261.2 | 274.4
IPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 77.3 | 72.5
IPL [Member] | EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 57.2 | 31.6
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 13.3 | 21.8
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 24.3 | 25.5
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 54.2 | 55.3
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 444.8 | 435.1
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 35.9 | 27.7
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 258.6 | 273.6
WPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 34.3 | 36.8
WPL [Member] | EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 53.2 | 32.2
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 13.0 | 23.5
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 49.8 | $ 41.3

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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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