XML 61 R51.htm IDEA: XBRL DOCUMENT
v3.19.3

Debt (Narrative) (Details) - USD ($) | 1 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Jul. 31, 2019 | Jun. 30, 2019 | Apr. 30, 2019 | Sep. 30, 2019 | Sep. 30, 2018
Debt [Line Items] |  |  |  |  |  |
Credit facility agreement, extension |  |  |  |  | 1 year |
Line of credit facility, current borrowing capacity | $ 1,000,000,000 |  |  |  | $ 1,000,000,000 |
Payments to retire long-term debt |  |  |  |  | 253,500,000 | $ 603,100,000
IPL [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Line of credit facility, current borrowing capacity | 250,000,000 |  |  |  | 250,000,000 |
Payments to retire long-term debt |  |  |  |  | 0 | 100,000,000.0
WPL [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Line of credit facility, current borrowing capacity | $ 300,000,000 |  |  |  | 300,000,000 |
Payments to retire long-term debt |  |  |  |  | $ 250,000,000.0 | $ 0
Senior Debentures [Member] | 3.6% senior debentures due 2029 [Member] | IPL [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Proceeds from issuance of long-term debt |  |  |  | $ 300,000,000 |  |
Interest rate, percent | 3.60% |  |  |  | 3.60% |
Senior Debentures [Member] | 3.5% senior debentures due 2049 [Member] | IPL [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Proceeds from issuance of long-term debt | $ 300,000,000 |  |  |  |  |
Interest rate, percent | 3.50% |  |  |  | 3.50% |
Debentures [Member] | 3% debentures due 2029 [Member] | WPL [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Proceeds from issuance of long-term debt |  |  | $ 350,000,000 |  |  |
Interest rate, percent | 3.00% |  |  |  | 3.00% |
Debentures [Member] | 5% debentures due 2019 [Member] | WPL [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Interest rate, percent |  |  | 5.00% |  |  |
Payments to retire long-term debt |  | $ 250,000,000 |  |  |  |
Parent Company [Member] |  |  |  |  |  |
Debt [Line Items] |  |  |  |  |  |
Line of credit facility, current borrowing capacity | $ 450,000,000 |  |  |  | $ 450,000,000 |

X
- Definition Agreement Term Of Extension + References No definition available. + Details
Name: | lnt_AgreementTermOfExtension
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceOfDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cash outflow for debt initially having maturity due 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 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
Name: | us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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: | us-gaap_LongtermDebtTypeAxis=lnt_SeniorDebenturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=lnt_ThreePointSixPercentSeniorDebenturesDueTwoThousandTwentyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=lnt_ThreePointFivePercentSeniorDebenturesDueTwoThousandFortyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_DebenturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=lnt_ThreePercentDebenturesDueTwoThousandTwentyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=lnt_FivePercentDebentureDueTwoThousandNineteenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |