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Summary Of Significant Accounting Policies (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 935.7 | $ 916.1
Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 411.3 | 408.7
IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 335.1 | 355.8
Energy conservation cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 61.3 | 55.1
IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 29.7 | 32.5
Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 39.2 | 17.7
Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 59.1 | 46.3
IPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 715.2 | 701.4
IPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 270.6 | 268.0
IPL [Member] | IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 335.1 | 355.8
IPL [Member] | Energy conservation cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 16.4 | 10.0
IPL [Member] | IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 29.7 | 32.5
IPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 25.4 | 5.2
IPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 38.0 | 29.9
WPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 220.5 | 214.7
WPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 140.7 | 140.7
WPL [Member] | IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 0 | 0
WPL [Member] | Energy conservation cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 44.9 | 45.1
WPL [Member] | IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 0 | 0
WPL [Member] | Commodity cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 13.8 | 12.5
WPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 21.1 | $ 16.4

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- 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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