v2.4.0.6

Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2012 IPL [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2012 Commodity Contracts [Member] | Jun. 30, 2012 Commodity Contracts [Member] | Dec. 31, 2011 Commodity Contracts [Member] | Sep. 30, 2011 Commodity Contracts [Member] | Jun. 30, 2011 Commodity Contracts [Member] | Dec. 31, 2010 Commodity Contracts [Member] | Sep. 30, 2012 Commodity Contracts [Member] IPL [Member] | Jun. 30, 2012 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2011 Commodity Contracts [Member] IPL [Member] | Sep. 30, 2011 Commodity Contracts [Member] IPL [Member] | Jun. 30, 2011 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2010 Commodity Contracts [Member] IPL [Member] | Sep. 30, 2012 Commodity Contracts [Member] WPL [Member] | Jun. 30, 2012 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2011 Commodity Contracts [Member] WPL [Member] | Sep. 30, 2011 Commodity Contracts [Member] WPL [Member] | Jun. 30, 2011 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2010 Commodity Contracts [Member] WPL [Member] | Sep. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] Alliant Energy [Member] | Sep. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] IPL [Member] | Sep. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] WPL [Member] | Sep. 30, 2012 Financial Transmission Rights [Member] Alliant Energy [Member] | Sep. 30, 2012 Financial Transmission Rights [Member] IPL [Member] | Sep. 30, 2012 Financial Transmission Rights [Member] WPL [Member]
Cumulative preferred stock rate | 8.375% | 4.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  | $ 17.6 | $ 18.8 | $ (0.9) | $ 11.9 | $ 18.1 | $ 2.8 | $ 15.5 | $ 14.1 | $ 4.3 | $ 12.4 | $ 17.5 | $ 4.3 | $ 2.1 | $ 4.7 | $ (5.2) | $ (0.5) | $ 0.6 | $ (1.5) | $ 5.6 | $ 1.2 | $ 4.4 | $ 23.2 | $ 16.7 | $ 6.5

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- Definition This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 157 -Paragraph 32 -Subparagraph c -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 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
Name: | us-gaap_PreferredStockDividendRatePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration