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v2.4.1.9

Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Debt Instrument [Line Items] |  |
Maximum amount outstanding (based on daily outstanding balances) | $ 353.8 us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod | $ 293.9 us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
Average amount outstanding (based on daily outstanding balances) | 255.9 us-gaap_ShorttermDebtAverageOutstandingAmount | 210.5 us-gaap_ShorttermDebtAverageOutstandingAmount
Weighted average interest rates | 0.20%lnt_WeightedAverageInterestRateShortTermDebt | 0.20%lnt_WeightedAverageInterestRateShortTermDebt
IPL [Member] |  |
Debt Instrument [Line Items] |  |
Maximum amount outstanding (based on daily outstanding balances) | 38.0 us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod / dei_LegalEntityAxis = lnt_IplMember | 26.3 us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod / dei_LegalEntityAxis = lnt_IplMember
Average amount outstanding (based on daily outstanding balances) | 0.2 us-gaap_ShorttermDebtAverageOutstandingAmount / dei_LegalEntityAxis = lnt_IplMember | 1.3 us-gaap_ShorttermDebtAverageOutstandingAmount / dei_LegalEntityAxis = lnt_IplMember
Weighted average interest rates | 0.20%lnt_WeightedAverageInterestRateShortTermDebt / dei_LegalEntityAxis = lnt_IplMember | 0.40%lnt_WeightedAverageInterestRateShortTermDebt / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Debt Instrument [Line Items] |  |
Maximum amount outstanding (based on daily outstanding balances) | 204.7 us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod / dei_LegalEntityAxis = lnt_WplMember | 190.0 us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod / dei_LegalEntityAxis = lnt_WplMember
Average amount outstanding (based on daily outstanding balances) | $ 122.9 us-gaap_ShorttermDebtAverageOutstandingAmount / dei_LegalEntityAxis = lnt_WplMember | $ 123.5 us-gaap_ShorttermDebtAverageOutstandingAmount / dei_LegalEntityAxis = lnt_WplMember
Weighted average interest rates | 0.10%lnt_WeightedAverageInterestRateShortTermDebt / dei_LegalEntityAxis = lnt_WplMember | 0.20%lnt_WeightedAverageInterestRateShortTermDebt / dei_LegalEntityAxis = lnt_WplMember

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- Definition Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of short-term debt outstanding by type or by instrument at that time. + References No definition available. + Details
Name: | lnt_WeightedAverageInterestRateShortTermDebt
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period. + References No definition available. + Details
Name: | us-gaap_ShorttermDebtAverageOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, the maximum amount borrowed at any time during the period. + References No definition available. + Details
Name: | us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
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: |