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Debt (Narrative) (Details) (USD $) | Dec. 31, 2014 revolving_credit_facility
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Debt Instrument [Line Items] |
Number of revolving credit facilities included in short-term borrowing arrangements (number of facilities) | 3 lnt_NumberOfShortTermDebtInstrumentsRevolvingCreditFacilities
Line of credit facility, current borrowing capacity | $ 1,000,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Non-recourse debt and hybrid securities consolidated capital maximum limit | 15.00%lnt_HybridSecuritiesMaximumLimit
Parent Company [Member] |
Debt Instrument [Line Items] |
Line of credit facility, current borrowing capacity | 300,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / dei_LegalEntityAxis = us-gaap_ParentCompanyMember
IPL [Member] |
Debt Instrument [Line Items] |
Line of credit facility, current borrowing capacity | 300,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |
Debt Instrument [Line Items] |
Line of credit facility, current borrowing capacity | $ 400,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / dei_LegalEntityAxis = lnt_WplMember

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- Definition Hybrid Securities Maximum Limit + References No definition available. + Details
Name: | lnt_HybridSecuritiesMaximumLimit
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of Short-Term Debt Instruments, Revolving Credit Facilities + References No definition available. + Details
Name: | lnt_NumberOfShortTermDebtInstrumentsRevolvingCreditFacilities
Namespace Prefix: | lnt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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

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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
Name: | us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
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: |

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