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Related Parties (Service Agreements) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Corporate Service Billings [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 148us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember	$ 140us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember	$ 129us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember
Corporate Service Billings [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	116us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember	103us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember	102us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_CorporateServiceBillingsMember
Sales Credited [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	8us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember	7us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember	10us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember
Sales Credited [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	6us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember	12us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember	14us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaSalesCreditedMember
Purchases Billed [Member] | IPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	422us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember	365us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember	301us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember
Purchases Billed [Member] | WPL [Member]
Related Party Transaction [Line Items]
Amounts billed between related parties	$ 125us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember	$ 68us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember	$ 61us-gaap_RelatedPartyTransactionAmountsOfTransaction / dei_LegalEntityAxis = lnt_WplMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_ScoaPurchasesBilledMember

X

- 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

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- Details

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X

- Details

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X

- Details

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