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v2.4.1.9

Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Assets: |  |
Derivative assets | $ 16.9 us-gaap_DerivativeFairValueOfDerivativeAsset | $ 38.6 us-gaap_DerivativeFairValueOfDerivativeAsset
Deferred proceeds (sales of receivables) | 117.7 lnt_FairValueOfDeferredProceedsOfReceivablesSold | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 4,490.4 us-gaap_DebtInstrumentFairValue | 4,418.2 us-gaap_DebtInstrumentFairValue
Cumulative preferred stock | 203.7 lnt_PreferredStockFairValue | 200.2 lnt_PreferredStockFairValue
Derivative liabilities | 42.7 us-gaap_DerivativeFairValueOfDerivativeLiability | 37.6 us-gaap_DerivativeFairValueOfDerivativeLiability
IPL [Member] |  |
Assets: |  |
Derivative assets | 6.5 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_IplMember | 28.0 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_IplMember
Deferred proceeds (sales of receivables) | 117.7 lnt_FairValueOfDeferredProceedsOfReceivablesSold / dei_LegalEntityAxis = lnt_IplMember | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / dei_LegalEntityAxis = lnt_IplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 2,092.9 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_IplMember | 2,053.0 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_IplMember
Cumulative preferred stock | 203.7 lnt_PreferredStockFairValue / dei_LegalEntityAxis = lnt_IplMember | 200.2 lnt_PreferredStockFairValue / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities | 20.0 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_IplMember | 19.5 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Assets: |  |
Derivative assets | 10.4 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_WplMember | 10.6 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_WplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,938.9 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_WplMember | 1,908.9 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities | 22.7 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_WplMember | 18.1 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_WplMember
Carrying Value [Member] |  |
Assets: |  |
Derivative assets | 16.9 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 38.6 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Deferred proceeds (sales of receivables) | 117.7 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,789.7 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 3,789.7 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Cumulative preferred stock | 200.0 us-gaap_PreferredStockValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 200.0 us-gaap_PreferredStockValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Derivative liabilities | 42.7 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 37.6 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Carrying Value [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 6.5 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 28.0 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Deferred proceeds (sales of receivables) | 117.7 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,768.8 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 1,768.7 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Cumulative preferred stock | 200.0 us-gaap_PreferredStockValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 200.0 us-gaap_PreferredStockValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities | 20.0 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 19.5 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Carrying Value [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 10.4 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember | 10.6 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,574.0 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember | 1,573.9 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities | 22.7 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember | 18.1 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember
Fair Value [Member] |  |
Assets: |  |
Derivative assets | 16.9 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 38.6 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Deferred proceeds (sales of receivables) | 117.7 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 4,490.4 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 4,418.2 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Cumulative preferred stock | 203.7 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 200.2 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Derivative liabilities | 42.7 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 37.6 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 6.5 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 28.0 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Deferred proceeds (sales of receivables) | 117.7 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 177.2 lnt_FairValueOfDeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 2,092.9 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 2,053.0 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Cumulative preferred stock | 203.7 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 200.2 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities | 20.0 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 19.5 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Fair Value [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 10.4 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember | 10.6 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,938.9 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember | 1,908.9 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities | $ 22.7 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember | $ 18.1 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember

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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13476-108611 + Details
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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313 + Details
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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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