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Summary Of Significant Accounting Policies (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,582.6 | $ 1,612.4
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 781.9 | 770.7
Pension and other postretirement benefits costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 541.4 | 549.2
Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 62.9 | 62.4
Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 32.2 | 34.9
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.0 | 30.0
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 17.0 | 40.2
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 117.2 | 125.0
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,207.9 | 1,217.9
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 757.2 | 746.2
IPL [Member] | Pension and other postretirement benefits costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 275.6 | 279.3
IPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 38.5 | 38.6
IPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.7 | 30.3
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.0 | 30.0
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 6.3 | 16.3
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 72.6 | 77.2
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 374.7 | 394.5
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 24.7 | 24.5
WPL [Member] | Pension and other postretirement benefits costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 265.8 | 269.9
WPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 24.4 | 23.8
WPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 4.5 | 4.6
WPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 0 | 0
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 10.7 | 23.9
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 44.6 | $ 47.8

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

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets 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 9, 10 -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant