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Fair Value Measurements (Fair Value Measurements Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Commodity Contracts [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | $ 17.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  | $ 4.4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (18.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  | 38.4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |
Sales | (0.9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |
Settlements | (12.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember | [1] | (18.9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember | [1]
Ending balance | (13.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  | 23.9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at March 31 | (16.5)us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |  | 31.0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember |
Commodity Contracts [Member] | IPL [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 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 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (12.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |  | 4.2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |
Sales | (0.9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |
Settlements | (11.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | [1] | (11.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | [1]
Ending balance | (5.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |  | 7.8 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at March 31 | (10.7)us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |  | 3.1 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember |
Commodity Contracts [Member] | WPL [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | (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 |
Total net gains (losses) (realized/unrealized) included in changes in net assets | (5.6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |  | 34.2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |
Settlements | (1.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |  | (7.9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |
Ending balance | (8.1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |  | 16.1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at March 31 | (5.8)us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |  | 27.9 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember |
Deferred Proceeds [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 177.2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |  | 203.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |
Sales | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |
Settlements | (59.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember | [1] | (32.7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember | [1]
Ending balance | 117.7 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |  | 170.8 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at March 31 | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |  | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember |
Deferred Proceeds [Member] | IPL [Member] |  |  |  |
Fair Value, Assets and Liabilities, Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | 177.2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 203.5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Total net gains (losses) (realized/unrealized) included in changes in net assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Sales | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
Settlements | (59.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember | [1] | (32.7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember | [1]
Ending balance | 117.7 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | 170.8 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at March 31 | $ 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |  | $ 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesChangesInAssetsAndLiabilitiesContinuedToBeHeldAmount / us-gaap_FairValueByAssetClassAxis = lnt_DeferredProceedsOfReceivablesSoldMember / dei_LegalEntityAxis = lnt_IplMember |
[1] | Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash proceeds received from the receivables sold.

X
- Definition The portion (amount) of unrealized gains and losses recognized as changes in net assets during the period and attributable to alternative investments continued to be held at period end. These gains or losses are attributable to certain investments for which net asset value per share is calculated for purposes of measuring fair value and which are subject to unobservable inputs. + 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 d -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized in the income statement for 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)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of sale 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)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of settlement 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)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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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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