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v2.4.1.9

Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Assets: |  |
Derivative assets | $ 33.0 us-gaap_DerivativeAssets | $ 23.5 us-gaap_DerivativeAssets
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,802.8 us-gaap_LongTermDebt | 3,348.4 us-gaap_LongTermDebt
Long-term debt (including current maturities) | 4,418.2 us-gaap_DebtInstrumentFairValue | 3,712.3 us-gaap_DebtInstrumentFairValue
Cumulative preferred stock | 200.2 lnt_PreferredStockFairValue | 167.0 lnt_PreferredStockFairValue
Derivative liabilities | 32.0 us-gaap_DerivativeLiabilities | 17.6 us-gaap_DerivativeLiabilities
IPL [Member] |  |
Assets: |  |
Derivative assets | 24.7 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_IplMember | 19.5 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_IplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,775.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember | 1,563.4 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember
Long-term debt (including current maturities) | 2,053.0 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_IplMember | 1,726.4 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_IplMember
Cumulative preferred stock | 200.2 lnt_PreferredStockFairValue / dei_LegalEntityAxis = lnt_IplMember | 167.0 lnt_PreferredStockFairValue / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities | 16.2 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_IplMember | 3.6 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Assets: |  |
Derivative assets | 8.3 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_WplMember | 4.0 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_WplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,580.6 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember | 1,339.1 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember
Long-term debt (including current maturities) | 1,908.9 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_WplMember | 1,532.9 us-gaap_DebtInstrumentFairValue / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities | 15.8 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_WplMember | 14.0 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_WplMember
Carrying Amount [Member] |  |
Assets: |  |
Derivative assets | 38.6 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 26.7 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Deferred proceeds (sales of receivables) | 177.2 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 203.5 lnt_DeferredProceedsOfReceivablesSold / 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 | 3,336.3 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 | 37.6 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember | 20.8 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember
Carrying Amount [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 28.0 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 21.1 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Deferred proceeds (sales of receivables) | 177.2 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 203.5 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,768.7 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 1,558.4 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 | 19.5 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember | 5.2 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_IplMember
Carrying Amount [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 10.6 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember | 5.6 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,573.9 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember | 1,332.1 us-gaap_LongTermDebt / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities | 18.1 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember | 15.6 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_CarryingValueMember / dei_LegalEntityAxis = lnt_WplMember
Fair Value [Member] |  |
Assets: |  |
Derivative assets | 38.6 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 26.7 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Deferred proceeds (sales of receivables) | 177.2 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 203.5 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 4,418.2 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 3,712.3 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Cumulative preferred stock | 200.2 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 167.0 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Derivative liabilities | 37.6 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember | 20.8 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 28.0 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 21.1 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Deferred proceeds (sales of receivables) | 177.2 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 203.5 lnt_DeferredProceedsOfReceivablesSold / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 2,053.0 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 1,726.4 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Cumulative preferred stock | 200.2 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 167.0 lnt_PreferredStockFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities | 19.5 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember | 5.2 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_IplMember
Fair Value [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 10.6 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember | 5.6 us-gaap_DerivativeAssets / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,908.9 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember | 1,532.9 us-gaap_DebtInstrumentFairValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities | $ 18.1 us-gaap_DerivativeLiabilities / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = lnt_FairValueMember / dei_LegalEntityAxis = lnt_WplMember | $ 15.6 us-gaap_DerivativeLiabilities / 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, after the 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 not subject to a master netting arrangement and not elected to be offset. + 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 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset. + 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 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + 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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