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Derivative Instruments (Gains And Losses From Derivative Instruments) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Regulatory Assets [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | $ (27.4) | $ 8.3 | $ (69.5) | $ 13.8
Regulatory Assets [Member] | IPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (9.5) | 7.3 | (33.3) | 8.7
Regulatory Assets [Member] | WPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (17.9) | 1.0 | (36.2) | 5.1
Regulatory Liabilities [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (2.5) | (6.2) | (4.9) | 63.2
Regulatory Liabilities [Member] | IPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (1.2) | (2.0) | 1.0 | 13.9
Regulatory Liabilities [Member] | WPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | $ (1.3) | $ (4.2) | $ (5.9) | $ 49.3

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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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