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Commitments And Contingencies (Narrative) (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2018 USD ($)
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | $ 2,278 |
Present value abandonment obligation | 34 |
IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 1,603 |
Indemnification obligations, maximum | 17 |
WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 661 |
Capital Purchase Obligation [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 95 |
Capital Purchase Obligation [Member] | IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 12 |
Capital Purchase Obligation [Member] | WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 83 |
Purchased Power [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 1,175 | [1]
Purchased Power [Member] | IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 1,127 | [1]
Purchased Power [Member] | WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 48 | [1]
Indemnification Agreement [Member] |  |
Commitments and Contingencies [Line Items] |  |
Indemnification obligations, maximum | 98 |
Indemnification Agreement [Member] | Purchased Power [Member] |  |
Commitments and Contingencies [Line Items] |  |
Indemnification obligations, maximum | $ 17 |
[1] | Includes payments required by purchased power agreements for capacity rights and minimum quantities of MWhs required to be purchased.

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- 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

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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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

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- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_IndemnificationGuaranteeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |