XML 95 R61.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Property, Plant and Equipment (Narrative) (Details) $ / shares in Units, $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 MW | Dec. 31, 2018 USD ($) MW | Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($) $ / shares | Dec. 31, 2015 USD ($) | Apr. 30, 2017 USD ($)
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  |  |  | $ 962.2 | $ 1,226.8 |  |
Allowance for funds used during construction |  |  |  | 49.7 | 62.5 | $ 36.9 |
Asset valuation charges, pre-tax |  |  |  | 0.0 | 86.4 | 0.0 |
Non-utility Generation, net | [1] |  |  | 90.9 | 135.0 |  |
IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  |  |  | 424.4 | 968.1 |  |
Allowance for funds used during construction |  |  |  | $ 31.4 | 52.0 | 28.2 |
Net assets acquired |  |  |  |  |  |  | $ 32.0
Wholesale power supply agreement termination notice term |  |  |  | 5 years |  |  |
Wholesale power supply agreement term |  |  |  | 10 years |  |  |
WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  |  |  | $ 537.8 | 258.7 |  |
Allowance for funds used during construction |  |  |  | $ 18.3 | 10.5 | 8.7 |
Corporate Services [Member] | Minimum [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  | 5 years |  |  |
Corporate Services [Member] | Maximum [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  | 30 years |  |  |
Marshalltown Generating Station [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  |  |  | 706 |  |  |
Marshalltown Generating Station [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric plant in service |  |  |  | $ 645.0 |  |  |
Construction work in progress |  |  |  |  | 612.0 |  |
Allowance for funds used during construction |  |  |  | $ 81.0 | 68.0 |  |
West Riverside Energy Center [Member] | WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  |  |  | 730 |  |  |
West Riverside Energy Center [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  |  |  | $ 339.0 | 81.0 |  |
Allowance for funds used during construction |  |  |  | $ 14.0 | 2.0 |  |
Expansion of Wind Generation [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  |  |  | 500 |  |  |
Expansion of Wind Generation [Member] | WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  |  |  | 129 |  |  |
Expansion of Wind Generation [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Construction work in progress |  |  |  | $ 264.0 | 102.0 |  |
Allowance for funds used during construction |  |  |  | $ 11.0 | 1.0 |  |
IPL (FERC formula) [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Authorized return on common equity |  |  |  | 10.97% |  |  |
Franklin County Wind Farm [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Asset valuation charges, pre-tax |  |  |  |  | 86.0 |  |
Asset valuation charges, after-tax |  |  |  |  | $ 51.0 |  |
Asset valuation charges, after-tax (in dollars per share) | $ / shares |  |  |  |  | $ 0.23 |  |
Gas - distribution [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Proceeds from asset sales |  |  |  |  |  | 11.0 |
Receipt of promissory note |  |  |  |  |  | 2.0 |
Gas - distribution [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Loss on sale of Minnesota distribution assets |  |  |  |  |  | 3.0 |
Electric - distribution [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Proceeds from asset sales |  |  |  |  |  | 129.0 |
Electric - distribution [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Loss on sale of Minnesota distribution assets |  |  |  |  |  | $ 11.0 |
Sheboygan Falls Energy Facility [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  | 35 years |  |  |
Non-utility Generation, net |  |  |  | $ 91.0 |  |  |
Customer Billing And Information System [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  | 12 years |  |  |
Software [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  | 5 years |  |  |
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | IPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  | 1,000 |  |  |  |  |
Scenario, Forecast [Member] | Expansion of Wind Generation [Member] | WPL [Member] |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  |  | 55 |  |  |  |
Net assets acquired |  |  | $ 74.0 |  |  |  |
[1] | Less accumulated depreciation of $50.5 million and $46.5 million for Alliant Energy as of December 31, 2017 and 2016, respectively. Refer to “Franklin County Wind Farm” below for discussion of the April 2017 transfer of the Franklin County wind farm from AEF to IPL pursuant to a February 2017 FERC order.

X
- Definition Asset valuation charges, after tax + References No definition available. + Details
Name: | lnt_Assetvaluationchargesaftertax
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Authorized Return On Common Equity + References No definition available. + Details
Name: | lnt_AuthorizedReturnOnCommonEquity
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Electric Capacity Of Wind Project + References No definition available. + Details
Name: | lnt_ElectricCapacityOfWindProject
Namespace Prefix: | lnt_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Fossil Fueled Energy Generating Units Capacity + References No definition available. + Details
Name: | lnt_FossilFueledEnergyGeneratingUnitsCapacity
Namespace Prefix: | lnt_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Carrying amount as of the balance sheet date of long-lived, depreciable Non-regulated Generation plant. + References No definition available. + Details
Name: | lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Wholesale Power Supply Agreement Term (in number of years) + References No definition available. + Details
Name: | lnt_WholesalePowerSupplyAgreementTerm
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Wholesale Power Supply Agreement Termination Notice Minimum Term + References No definition available. + Details
Name: | lnt_WholesalePowerSupplyAgreementTerminationNoticeTerm
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=73531372&loc=d3e2207-128464 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472 + Details
Name: | us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of gain (loss) from the difference between the sale price or salvage price and the book value of an asset that was sold or retired, and gain (loss) from the write down of assets from their carrying value to fair value. + References No definition available. + Details
Name: | us-gaap_GainLossOnSalesOfAssetsAndAssetImpairmentCharges
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition After tax earnings per share or per unit impact arising from asset impairment. + References No definition available. + Details
Name: | us-gaap_ImpairmentEffectOnEarningsPerShareAfterTax
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 958 -SubTopic 225 -Section 45 -Paragraph 11 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68048823&loc=d3e92212-112881 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Impairment -URI http://asc.fasb.org/extlink&oid=6515133 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2921-110230 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228 + Details
Name: | us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of increase (decrease) in noncurrent assets classified as other. + References No definition available. + Details
Name: | us-gaap_IncreaseDecreaseInOtherNoncurrentAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 + Details
Name: | us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
Name: | us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Period end amount of construction work in progress in public utility. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Period end amount of total gross PPE. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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: | dei_LegalEntityAxis=lnt_CorporateServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
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: |

X
- Details
Name: | us-gaap_PublicUtilityAxis=lnt_MarshalltownGeneratingStationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_PublicUtilityAxis=lnt_RiversideEnergyCenterMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_PublicUtilityAxis=lnt_ExpansionofWindGenerationProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilityAxis=lnt_IplFercFormulaOtherProjectsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=lnt_FranklinCountyWindProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_GasDistributionEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ElectricDistributionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=lnt_SheboyganFallsEnergyFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=lnt_CustomerBillingAndInformationSystemMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_SoftwareAndSoftwareDevelopmentCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |