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Derivative Instruments (Gains And Losses From Derivative Instruments) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in unrealized gains (losses) from derivative instruments	$ (13.8)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember	$ (14.7)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember	$ (37.9)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember
Regulatory Assets [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in unrealized gains (losses) from derivative instruments	(5.8)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	(6.6)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	(16.8)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
Regulatory Assets [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in unrealized gains (losses) from derivative instruments	(8.0)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	(8.1)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	(21.1)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryAssetsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in unrealized gains (losses) from derivative instruments	37.4us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember	22.2us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember	20.3us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember
Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in unrealized gains (losses) from derivative instruments	10.2us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	11.8us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	13.5us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Changes in unrealized gains (losses) from derivative instruments	$ 27.2us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	$ 10.4us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	$ 6.8us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_BalanceSheetLocationAxis = lnt_RegulatoryLiabilitiesMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / 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

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