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Regulatory Matters (Customer Billing Credits) (Details) (IPL [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended | 36 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2016
Regulatory Liabilities [Line Items] |  |  |  |
Billing credits to reduce retail electric customers' bills | $ 72 lnt_CustomerBillingCredits |  |  |
Scenario, Forecast [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Billing credits to reduce retail electric customers' bills |  | 10 lnt_CustomerBillingCredits / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember | 25 lnt_CustomerBillingCredits / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember | 105 lnt_CustomerBillingCredits / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Target [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Billing credits to reduce retail electric customers' bills | $ 70 lnt_CustomerBillingCredits / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TargetMember |  |  |

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- Definition Customer Billing Credits + References No definition available. + Details
Name: | lnt_CustomerBillingCredits
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Details
Name: | us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TargetMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |