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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
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Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |
Fair value, net derivative assets | $ 17.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue | $ 4.4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue | $ 11.9 us-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 | 24.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember | 18.3 us-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 | 19.4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 14.6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 12.5 us-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 | 22.6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 16.7 us-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 | $ 2.3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EnergyAxis = lnt_FinancialTransmissionRightsMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | $ 1.6 us-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) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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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 + Details
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