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Related Parties (Service Agreements) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Corporate Services Billings [Member] | IPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | $ 36 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember | $ 33 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember
Corporate Services Billings [Member] | WPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | 28 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember | 28 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember
Sales Credited [Member] | IPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | 4 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember | 2 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember
Sales Credited [Member] | WPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | 6 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember | (1)us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember
Purchases Billed [Member] | IPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | 79 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember | 99 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember
Purchases Billed [Member] | WPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | $ 14 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember | $ 30 us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember

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- Definition Amount of transactions with related party during the financial reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 + Details
Name: | us-gaap_RelatedPartyTransactionAmountsOfTransaction
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=lnt_CorporateServiceBillingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=lnt_ScoaPurchasesBilledMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=lnt_ScoaSalesCreditedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |