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Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 6 Months Ended |  | 3 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 Alliant Energy and IPL [Member] IPL's tax benefit riders [Member] | Jun. 30, 2013 IPL [Member] | Jun. 30, 2012 IPL [Member] | Jun. 30, 2013 IPL [Member] Electric tax benefit rider [Member] | Jun. 30, 2013 IPL [Member] Gas tax benefit rider [Member] | Jun. 30, 2013 WPL [Member] | Jun. 30, 2012 WPL [Member] | Jun. 30, 2013 Wpl's Edgewater Unit 5 Purchase [Member] WPL [Member] | Jun. 30, 2012 2013 and 2014 Test Period Retail Electric And Gas Final Order [Member] Alliant Energy and WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities | $ (15.1) | $ (43.5) | $ 43.0 | $ (27.1) | $ (61.5) | $ 38.0 | $ 5.0 | $ 12.0 | $ 18.0 |  |
Jointly owned utility plant, proportionate ownership share |  |  |  |  |  |  |  |  |  | 25.00% |
Increase in regulatory assets |  |  |  |  |  |  |  |  |  |  | 5
Credit to utility other operation and maintenance |  |  |  |  |  |  |  |  |  |  | $ 5

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- Definition The increase (decrease) during the reporting period in the value of other assets that are created when regulatory agencies permits public utilities to defer costs to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The proportionate share of interest (percent) in a jointly owned utility plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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