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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,917.2 | $ 1,908.6
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,002.7 | 987.7
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 570.0 | 579.5
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 96.1 | 92.4
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 86.6 | 70.6
WPL's EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 43.2 | 45.0
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.7 | 26.9
Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.3 | 35.9
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 65.6 | 70.6
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,405.9 | 1,402.6
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 972.0 | 958.2
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 293.6 | 298.1
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 54.1 | 50.8
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 25.5 | 28.2
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.7 | 26.9
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 0.3 | 2.8
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 33.7 | 37.6
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 511.3 | 506.0
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.7 | 29.5
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 276.4 | 281.4
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 42.0 | 41.6
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 61.1 | 42.4
WPL [Member] | WPL's EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 43.2 | 45.0
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.0 | 33.1
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 31.9 | $ 33.0

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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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