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Commitments And Contingencies (Other Purchase Obligations) (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2018 USD ($)
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | $ 2,278 |
Individual commitments incurred | 1 |
IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 1,603 |
Individual commitments incurred | 1 |
WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 661 |
Individual commitments incurred | 1 |
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]
Natural gas [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 892 |
Natural gas [Member] | IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 362 |
Natural gas [Member] | WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 530 |
Coal [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 165 | [2]
Coal [Member] | IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 88 | [2]
Coal [Member] | WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 77 | [2]
Other [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 46 | [3]
Other [Member] | IPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | 26 | [3]
Other [Member] | WPL [Member] |  |
Commitments and Contingencies [Line Items] |  |
Minimum future commitments | $ 6 | [3]
[1] | Includes payments required by purchased power agreements for capacity rights and minimum quantities of MWhs required to be purchased.
[2] | Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments. These commitments were assigned to IPL and WPL based on information available as of March 31, 2018 regarding expected future usage, which is subject to change.
[3] | Includes individual commitments incurred during the normal course of business that exceeded $1 million at March 31, 2018.

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- Definition Individual commitments incurred in excess of $1 million + References No definition available. + Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
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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_PurchasedPowerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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