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Derivative Instruments (Gains And Losses From Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Regulatory Assets [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from commodity derivative instruments | $ (20.9)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember | $ 10.1 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember
Regulatory Assets [Member] | IPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from commodity derivative instruments | (11.7)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / dei_LegalEntityAxis = lnt_IplMember | 5.5 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / dei_LegalEntityAxis = lnt_IplMember
Regulatory Assets [Member] | WPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from commodity derivative instruments | (9.2)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / dei_LegalEntityAxis = lnt_WplMember | 4.6 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / dei_LegalEntityAxis = lnt_WplMember
Regulatory Liabilities [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from commodity derivative instruments | 2.2 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember | 48.1 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember
Regulatory Liabilities [Member] | IPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from commodity derivative instruments | (1.4)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / dei_LegalEntityAxis = lnt_IplMember | 12.1 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / dei_LegalEntityAxis = lnt_IplMember
Regulatory Liabilities [Member] | WPL [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Gains (losses) from commodity derivative instruments | $ 3.6 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / dei_LegalEntityAxis = lnt_WplMember | $ 36.0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / dei_LegalEntityAxis = lnt_WplMember

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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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