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Derivative Instruments (Fair Value Of Financial Instruments) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Derivatives, Fair Value [Line Items] |  |
Current derivative assets | $ 30.5 us-gaap_DerivativeAssetsCurrent | $ 25.6 us-gaap_DerivativeAssetsCurrent
Non-current derivative assets | 8.1 us-gaap_DerivativeAssetsNoncurrent | 1.1 us-gaap_DerivativeAssetsNoncurrent
Current derivative liabilities | 28.1 us-gaap_DerivativeLiabilitiesCurrent | 6.7 us-gaap_DerivativeLiabilitiesCurrent
Non-current derivative liabilities | 9.5 us-gaap_DerivativeLiabilitiesNoncurrent | 14.1 us-gaap_DerivativeLiabilitiesNoncurrent
IPL [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Current derivative assets | 27.4 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 20.2 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
Non-current derivative assets | 0.6 us-gaap_DerivativeAssetsNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 0.9 us-gaap_DerivativeAssetsNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
Current derivative liabilities | 16.4 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 3.0 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
Non-current derivative liabilities | 3.1 us-gaap_DerivativeLiabilitiesNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember | 2.2 us-gaap_DerivativeLiabilitiesNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Derivatives, Fair Value [Line Items] |  |
Current derivative assets | 3.1 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | 5.4 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
Non-current derivative assets | 7.5 us-gaap_DerivativeAssetsNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | 0.2 us-gaap_DerivativeAssetsNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
Current derivative liabilities | 11.7 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | 3.7 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember
Non-current derivative liabilities | $ 6.4 us-gaap_DerivativeLiabilitiesNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember | $ 11.9 us-gaap_DerivativeLiabilitiesNoncurrent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = lnt_WplMember

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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, expected to be settled within one year or normal operating cycle, if longer. 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 -Glossary Current Assets -URI http://asc.fasb.org/extlink&oid=6509628 + Details
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