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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,908.6 | $ 1,783.7
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 987.7 | 955.3
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 579.5 | 570.2
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 92.4 | 73.7
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 70.6 | 46.9
WPL's EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 45.0 | 0.0
Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 35.9 | 31.1
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.9 | 27.4
Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 16.4 | 16.0
Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 14.3 | 16.1
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.9 | 47.0
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,402.6 | 1,357.9
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 958.2 | 928.0
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 298.1 | 287.9
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 50.8 | 41.4
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 28.2 | 28.0
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 2.8 | 0.4
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.9 | 27.4
IPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 11.0 | 11.5
IPL [Member] | Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 9.5 | 10.9
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 17.1 | 22.4
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 506.0 | 425.8
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.5 | 27.3
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 281.4 | 282.3
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 41.6 | 32.3
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 42.4 | 18.9
WPL [Member] | WPL's EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 45.0 | 0.0
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 33.1 | 30.7
WPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 5.4 | 4.5
WPL [Member] | Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 4.8 | 5.2
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 22.8 | $ 24.6

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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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