XML 61 R38.htm IDEA: XBRL DOCUMENT
v3.20.2

Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,843.0 | $ 1,844.7
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 842.5 | 817.6
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 501.2 | 524.0
Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 122.9 | 134.0
Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 114.0 | 111.8
IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 109.4 | 108.2
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 47.0 | 39.5
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 19.8 | 21.1
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 86.2 | 88.5
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,404.9 | 1,399.3
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 797.4 | 776.8
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 251.3 | 262.5
IPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 81.8 | 87.9
IPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 78.2 | 76.2
IPL [Member] | IPL's DAEC PPA amendment [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 109.4 | 108.2
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 20.5 | 18.3
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 19.8 | 21.1
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 46.5 | 48.3
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 438.1 | 445.4
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 45.1 | 40.8
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 249.9 | 261.5
WPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 41.1 | 46.1
WPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 35.8 | 35.6
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.5 | 21.2
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 39.7 | $ 40.2

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

X
- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_DeferredIncomeTaxChargesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_PensionAndOtherPostretirementPlansCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=lnt_AssetsRetiredEarlyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_AssetRetirementObligationCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=lnt_DAECPPAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_DeferredDerivativeGainLossMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=lnt_EmissionAllowancesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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: |