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Derivative Instruments (Gains And Losses From Derivative Instruments) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Regulatory Assets [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | $ (21.2) | $ (4.6) | $ (42.1) | $ 5.5
Regulatory Assets [Member] | IPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (12.1) | (4.1) | (23.8) | 1.4
Regulatory Assets [Member] | WPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (9.1) | (0.5) | (18.3) | 4.1
Regulatory Liabilities [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | (4.6) | 21.3 | (2.4) | 69.4
Regulatory Liabilities [Member] | IPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | 3.6 | 3.8 | 2.2 | 15.9
Regulatory Liabilities [Member] | WPL [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Gains (losses) from commodity derivative instruments | $ (8.2) | $ 17.5 | $ (4.6) | $ 53.5

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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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