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Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 17.9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue	$ 4.4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue	$ 11.9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative liabilities	(7.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember	(13.9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember
FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	24.9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember	18.3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember
IPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	19.4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	14.6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	12.5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative liabilities	(3.2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	(2.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	22.6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember	16.7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
WPL [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	(1.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	(10.2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	(0.6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Excluding FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative liabilities	(3.8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	(11.8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_EnergyAxis = lnt_ExcludingFinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | FTRs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, net derivative assets	$ 2.3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember	$ 1.6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

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- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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