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Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 744.1 | $ 821.2
Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 406.2 | 421.7
IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 179.5 | 243.0
Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 53.0 | 64.3
Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 33.8 | 15.4
Electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 31.4 | 19.4
Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 40.2 | 57.4
IPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 502.5 | 583.5
IPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 261.3 | 279.1
IPL [Member] | IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 179.5 | 243.0
IPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 0.0 | 0.0
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 22.9 | 15.1
IPL [Member] | Electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 15.8 | 19.4
IPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 23.0 | 26.9
WPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 241.6 | 237.7
WPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 144.9 | 142.6
WPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 53.0 | 64.3
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 10.9 | 0.3
WPL [Member] | Electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 15.6 | 0.0
WPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 17.2 | $ 30.5

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities 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 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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