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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,807.9 | $ 1,783.7
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 976.5 | 955.3
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 548.4 | 570.2
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 81.8 | 73.7
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 68.4 | 46.9
Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 33.2 | 31.1
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.5 | 27.4
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 73.1 | 79.1
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,370.2 | 1,357.9
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 947.3 | 928.0
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 278.6 | 287.9
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 47.6 | 41.4
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.3 | 28.0
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1.2 | 0.4
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.5 | 27.4
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.7 | 44.8
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 437.7 | 425.8
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.2 | 27.3
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 269.8 | 282.3
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 34.2 | 32.3
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.1 | 18.9
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 32.0 | 30.7
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 33.4 | $ 34.3

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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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