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Debt (Other Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Debt Instrument [Line Items] |  |  |  |
Maximum amount outstanding (based on daily outstanding balances) | $ 223.1 | $ 162.8 | $ 243.4 | $ 162.8
Average amount outstanding (based on daily outstanding balances) | 191.4 | 100.8 | 181.2 | 83.6
Weighted average interest rates | 0.20% | 0.40% | 0.30% | 0.40%
IPL [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum amount outstanding (based on daily outstanding balances) | 6.0 | 30.3 | 26.3 | 35.4
Average amount outstanding (based on daily outstanding balances) | 0.2 | 7.6 | 2.5 | 10.2
Weighted average interest rates | 0.30% | 0.40% | 0.40% | 0.40%
WPL [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Maximum amount outstanding (based on daily outstanding balances) | 159.4 | 35.6 | 160.0 | 35.6
Average amount outstanding (based on daily outstanding balances) | $ 125.7 | $ 14.9 | $ 99.4 | $ 14.0
Weighted average interest rates | 0.20% | 0.30% | 0.20% | 0.20%

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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

X
- 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