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Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 711.0 | $ 737.7
Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 408.1 | 406.0
IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 141.1 | 159.2
Electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 51.4 | 43.5
Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 41.6 | 48.3
Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 35.2 | 37.6
Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 33.6 | 43.1
IPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 468.2 | 489.2
IPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 263.1 | 260.4
IPL [Member] | IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 141.1 | 159.2
IPL [Member] | Electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 25.5 | 21.9
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 | 20.4 | 23.5
IPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 18.1 | 24.2
WPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 242.8 | 248.5
WPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 145.0 | 145.6
WPL [Member] | Electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 25.9 | 21.6
WPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 41.6 | 48.3
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 14.8 | 14.1
WPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 15.5 | $ 18.9

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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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