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Derivative Instruments (Credit Risk Related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Derivative [Line Items] |  |
Aggregate fair value | $ 37.6 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue | $ 20.8 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Credit support to be posted if triggered | 37.4 us-gaap_AdditionalCollateralAggregateFairValue | 20.8 us-gaap_AdditionalCollateralAggregateFairValue
IPL [Member] |  |
Derivative [Line Items] |  |
Aggregate fair value | 19.5 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue / dei_LegalEntityAxis = lnt_IplMember | 5.2 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue / dei_LegalEntityAxis = lnt_IplMember
Credit support to be posted if triggered | 19.5 us-gaap_AdditionalCollateralAggregateFairValue / dei_LegalEntityAxis = lnt_IplMember | 5.2 us-gaap_AdditionalCollateralAggregateFairValue / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Derivative [Line Items] |  |
Aggregate fair value | 18.1 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue / dei_LegalEntityAxis = lnt_WplMember | 15.6 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue / dei_LegalEntityAxis = lnt_WplMember
Credit support to be posted if triggered | $ 17.9 us-gaap_AdditionalCollateralAggregateFairValue / dei_LegalEntityAxis = lnt_WplMember | $ 15.6 us-gaap_AdditionalCollateralAggregateFairValue / dei_LegalEntityAxis = lnt_WplMember

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- Definition The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
Name: | us-gaap_AdditionalCollateralAggregateFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
Name: | us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |