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Commitments And Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015
Commitments and Contingencies [Line Items] |
Minimum future commitments | $ 2,283 us-gaap_LongTermPurchaseCommitmentAmount
Performance guarantees outstanding | 251 lnt_PerformanceGuaranteesOutstanding
Present value abandonment obligation | 35 lnt_PresentValueAbandonmentObligation
IPL [Member] |
Commitments and Contingencies [Line Items] |
Minimum future commitments | 1,795 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_IplMember
Number of sites monitoring and remediating (in sites) | 25 lnt_Numberofsitesremediatingandmonitoring / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |
Commitments and Contingencies [Line Items] |
Minimum future commitments | 488 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_WplMember
Number of sites monitoring and remediating (in sites) | 5 lnt_Numberofsitesremediatingandmonitoring / dei_LegalEntityAxis = lnt_WplMember
Capital Purchase Obligation [Member] |
Commitments and Contingencies [Line Items] |
Minimum future commitments | 26 us-gaap_LongTermPurchaseCommitmentAmount / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = lnt_CapitalPurchaseObligationMember
Capital Purchase Obligation [Member] | IPL [Member] |
Commitments and Contingencies [Line Items] |
Minimum future commitments | 2 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = lnt_CapitalPurchaseObligationMember
Capital Purchase Obligation [Member] | WPL [Member] |
Commitments and Contingencies [Line Items] |
Minimum future commitments | 24 us-gaap_LongTermPurchaseCommitmentAmount / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = lnt_CapitalPurchaseObligationMember
Performance guarantees outstanding, 2015 [Member] |
Commitments and Contingencies [Line Items] |
Performance guarantees outstanding | 128 lnt_PerformanceGuaranteesOutstanding / us-gaap_LossContingenciesByNatureOfContingencyAxis = lnt_PerformanceGuaranteesOutstandingYearOneMember
Performance guarantees outstanding, 2016 [Member] |
Commitments and Contingencies [Line Items] |
Performance guarantees outstanding | 48 lnt_PerformanceGuaranteesOutstanding / us-gaap_LossContingenciesByNatureOfContingencyAxis = lnt_PerformanceGuaranteesOutstandingYearTwoMember
Performance guarantees outstanding, 2017 [Member] |
Commitments and Contingencies [Line Items] |
Performance guarantees outstanding | 75 lnt_PerformanceGuaranteesOutstanding / us-gaap_LossContingenciesByNatureOfContingencyAxis = lnt_PerformanceGuaranteesOutstandingYearThreeMember
Environmental Issue [Member] | WPL [Member] |
Commitments and Contingencies [Line Items] |
Environmental mitigation projects to be completed, value | $ 7 us-gaap_LossContingencyDamagesAwardedValue / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LitigationCaseAxis = us-gaap_EnvironmentalIssueMember
Minimum [Member] |
Commitments and Contingencies [Line Items] |
Warranty period | 12 months
Maximum [Member] |
Commitments and Contingencies [Line Items] |
Warranty period | 60 months

X
- Definition Number of sites remediating and monitoring + References No definition available. + Details
Name: | lnt_Numberofsitesremediatingandmonitoring
Namespace Prefix: | lnt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Performance Guarantees Outstanding + References No definition available. + Details
Name: | lnt_PerformanceGuaranteesOutstanding
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Present value abandonment obligation + References No definition available. + Details
Name: | lnt_PresentValueAbandonmentObligation
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Warranty Period + References No definition available. + Details
Name: | lnt_WarrantyPeriod
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The minimum amount the entity agreed to spend under the long-term purchase commitment. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

X
- Definition Amount of damages awarded to the plaintiff in the legal matter. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
Name: | us-gaap_LossContingencyDamagesAwardedValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

X
- Details
Name: | us-gaap_LitigationCaseAxis=us-gaap_EnvironmentalIssueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=lnt_CapitalPurchaseObligationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=lnt_PerformanceGuaranteesOutstandingYearOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=lnt_PerformanceGuaranteesOutstandingYearThreeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=lnt_PerformanceGuaranteesOutstandingYearTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |