XML 82 R53.htm IDEA: XBRL DOCUMENT
v3.20.4

Regulatory Matters (Narrative) (Details) $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | Aug. 31, 2020 customer | Dec. 31, 2020 USD ($) MW | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2021 USD ($)
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Credits issued to customers |  |  | $ (113) | $ (40) | $ 1 |  |
Regulatory liabilities |  |  | 1,306 | 1,424 |  |  |
Electric transmission cost recovery [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  | 68 | 88 |  |  |
IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets not earning a return |  |  | 81 |  |  |  |
Credits issued to customers |  |  | $ (20) | 2 | 1 |  |
Electric capacity of wind farm (in megawatts) | MW |  |  | 1,000 |  |  |  |
Regulatory liabilities |  |  | $ 676 | 715 |  |  |
IPL [Member] | Electric transmission cost recovery [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Credits issued to customers |  |  | 42 |  |  |  |
Regulatory liabilities |  |  | 39 | 51 |  |  |
IPL [Member] | Derecho windstorm [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Number of customers without power, derecho windstorm (number of customers) | customer |  | 250,000 |  |  |  |  |
Derecho windstorm repair and rebuilding costs |  |  | 135 |  |  |  |
Net regulatory liability, derecho windstorm |  |  | 7 |  |  |  |
WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets not earning a return |  |  | 11 |  |  |  |
Credits issued to customers |  |  | (93) | (42) | 1 |  |
Regulatory liabilities |  |  | 630 | 709 |  |  |
WPL [Member] | Electric transmission cost recovery [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  | 29 | 37 |  |  |
2020 Test Period Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  | $ 90 |  |  |
Authorized increase (decrease) in final rates, amount |  |  | 127 |  |  |  |
2020 Test Period Retail Electric [Member] | IPL [Member] | Tax-related [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  | 27 |  |  |  |
2020 Test Period Retail Electric [Member] | IPL [Member] | Revenue Subject to Refund [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities |  |  | 8 |  |  |  |
2020 Test Period Retail Electric [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  | $ 29 |  |  |  |
Authorized increase (decrease) in final rates, percentage |  |  | 2.00% |  |  |  |
2020 Test Period Retail Gas [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  | $ 12 |  |  |  |
2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  | $ 102 |
Authorized increase (decrease) in final rates, amount |  |  |  |  | $ 130 |  |
2019 Test Period Retail Electric [Member] | WPL [Member] | Revenue Subject to Refund [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Credits issued to customers |  |  | $ 12 |  |  |  |
DAEC PPA [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Buyout payment | $ 110 |  |  |  |  |  |
PPA term amendment |  |  | 5 years |  |  |  |
Forecast [Member] | IPL [Member] | Derecho windstorm [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Derecho windstorm repair and rebuilding costs |  |  |  |  |  |  | $ 140

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 Long-term Contract for Purchase of Electric Power, Buyout Payment + References No definition available. + Details
Name: | lnt_LongtermContractforPurchaseofElectricPowerBuyoutPayment
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Long-term Contract for Purchase of Electric Power, Contract Term Amendment + References No definition available. + Details
Name: | lnt_LongtermContractforPurchaseofElectricPowerContractTermAmendment
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number Of Customers Without Power Derecho Windstorm + References No definition available. + Details
Name: | lnt_NumberOfCustomersWithoutPowerDerechoWindstorm
Namespace Prefix: | lnt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Regulatory Matters [Line Items] + References No definition available. + Details
Name: | lnt_RegulatoryMattersLineItems
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of regulatory costs not yet approved by the regulatory authority. + References No definition available. + Details
Name: | us-gaap_AmountOfRegulatoryCostsNotYetApproved
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of capitalized storm costs of a regulated entity that are expected to be recoverable through rate adjustments after one year or beyond the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557228&loc=d3e43603-110378 + Details
Name: | us-gaap_DeferredStormAndPropertyReserveDeficiencyNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesInterimRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities 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 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + 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 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121594574&loc=d3e44360-110383 + Details
Name: | us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=lnt_IplSElectricTransmissionServiceCostsMember
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: | us-gaap_RegulatoryLiabilityAxis=lnt_DerechoWindstormMember
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_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwoThousandTwentyTestPeriodRetailElectricMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_RevenueSubjectToRefundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwoThousandTwentyTestPeriodRetailGasMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwoThousandSixteenTestYearRetailElectricMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilitiesRegulatoryProceedingAxis=lnt_TwentyNineteenTestPeriodRetailElectricMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |