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Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Derivative Instruments [Line Items] |  |
Derivative assets, Gross (as reported) | $ 16.9 us-gaap_DerivativeFairValueOfDerivativeAsset | $ 38.6 us-gaap_DerivativeFairValueOfDerivativeAsset
Derivative assets, Net | 10.3 us-gaap_DerivativeAssets | 33.0 us-gaap_DerivativeAssets
Derivative liabilities, Gross (as reported) | 42.7 us-gaap_DerivativeFairValueOfDerivativeLiability | 37.6 us-gaap_DerivativeFairValueOfDerivativeLiability
Derivative liabilities, Net | 36.1 us-gaap_DerivativeLiabilities | 32.0 us-gaap_DerivativeLiabilities
IPL [Member] |  |
Derivative Instruments [Line Items] |  |
Derivative assets, Gross (as reported) | 6.5 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_IplMember | 28.0 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_IplMember
Derivative assets, Net | 5.1 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_IplMember | 24.7 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities, Gross (as reported) | 20.0 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_IplMember | 19.5 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_IplMember
Derivative liabilities, Net | 18.6 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_IplMember | 16.2 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Derivative Instruments [Line Items] |  |
Derivative assets, Gross (as reported) | 10.4 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_WplMember | 10.6 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = lnt_WplMember
Derivative assets, Net | 5.2 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_WplMember | 8.3 us-gaap_DerivativeAssets / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities, Gross (as reported) | 22.7 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_WplMember | 18.1 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = lnt_WplMember
Derivative liabilities, Net | $ 17.5 us-gaap_DerivativeLiabilities / dei_LegalEntityAxis = lnt_WplMember | $ 15.8 us-gaap_DerivativeLiabilities / 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. 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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