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Debt (Other Short-Term Borrowings) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Debt [Line Items] |  |
Maximum amount outstanding (based on daily outstanding balances) | $ 336.4 | $ 325.5
Average amount outstanding (based on daily outstanding balances) | $ 310.1 | $ 276.5
Weighted average interest rates | 1.90% | 0.90%
IPL [Member] |  |
Debt [Line Items] |  |
Maximum amount outstanding (based on daily outstanding balances) | $ 3.0 | $ 9.2
Average amount outstanding (based on daily outstanding balances) | $ 0.0 | $ 0.1
Weighted average interest rates | 1.80% | 1.10%
WPL [Member] |  |
Debt [Line Items] |  |
Maximum amount outstanding (based on daily outstanding balances) | $ 36.7 | $ 113.6
Average amount outstanding (based on daily outstanding balances) | $ 11.5 | $ 79.1
Weighted average interest rates | 1.60% | 0.70%

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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
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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
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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
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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
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |