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Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |  |  |  |  |  |  |  |
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Dec. 31, 2013 IPL [Member] | Dec. 31, 2012 IPL [Member] | Dec. 31, 2012 WPL [Member] | Dec. 31, 2013 Commodity Contracts [Member] | Dec. 31, 2012 Commodity Contracts [Member] | Dec. 31, 2011 Commodity Contracts [Member] | Dec. 31, 2013 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2012 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2011 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2013 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2012 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2011 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2013 Excluding FTRs [Member] Commodity Contracts [Member] | Dec. 31, 2013 Excluding FTRs [Member] Commodity Contracts [Member] IPL [Member] | Dec. 31, 2013 Excluding FTRs [Member] Commodity Contracts [Member] WPL [Member] | Dec. 31, 2013 FTRs [Member] Commodity Contracts [Member] | Dec. 31, 2013 FTRs [Member] Commodity Contracts [Member] IPL [Member] | Dec. 31, 2013 FTRs [Member] Commodity Contracts [Member] WPL [Member]
Cumulative preferred stock rate | 5.10% | 8.375% | 4.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value |  |  |  | $ 4.4 | $ 11.9 | $ (0.9) | $ 14.6 | $ 12.5 | $ 4.3 | $ (10.2) | $ (0.6) | $ (5.2) | $ (13.9) | $ (2.1) | $ (11.8) | $ 18.3 | $ 16.7 | $ 1.6

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- Definition Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: 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=25499696&loc=d3e19207-110258 + Details
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- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
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