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Derivative Instruments (Gains And Losses From Derivative Instruments Not Designated As Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Regulatory Assets [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from derivative instruments not designated as hedging instruments | $ 9.5 | $ (39.7)
Regulatory Assets [Member] | IPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from derivative instruments not designated as hedging instruments | 2.7 | (22.2)
Regulatory Assets [Member] | WPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from derivative instruments not designated as hedging instruments | 6.8 | (17.5)
Regulatory Liabilities [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from derivative instruments not designated as hedging instruments | 16.4 | 1.4
Regulatory Liabilities [Member] | IPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from derivative instruments not designated as hedging instruments | 8.8 | 0
Regulatory Liabilities [Member] | WPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from derivative instruments not designated as hedging instruments | $ 7.6 | $ 1.4

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- Definition Net gain (loss) from the increase (decrease) in fair value of derivative instrument not designated as hedging instrument. + 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=7476318&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 45 -Subparagraph b(1)(b) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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