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Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2013 IPL [Member] | Dec. 31, 2012 IPL [Member] | Dec. 31, 2012 WPL [Member] | Mar. 31, 2013 Commodity Contracts [Member] | Dec. 31, 2012 Commodity Contracts [Member] | Mar. 31, 2012 Commodity Contracts [Member] | Dec. 31, 2011 Commodity Contracts [Member] | Mar. 31, 2013 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2012 Commodity Contracts [Member] IPL [Member] | Mar. 31, 2012 Commodity Contracts [Member] IPL [Member] | Dec. 31, 2011 Commodity Contracts [Member] IPL [Member] | Mar. 31, 2013 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2012 Commodity Contracts [Member] WPL [Member] | Mar. 31, 2012 Commodity Contracts [Member] WPL [Member] | Dec. 31, 2011 Commodity Contracts [Member] WPL [Member] | Mar. 31, 2013 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] | Mar. 31, 2013 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] IPL [Member] | Mar. 31, 2013 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] WPL [Member] | Mar. 31, 2013 Financial Transmission Rights [Member] | Mar. 31, 2013 Financial Transmission Rights [Member] IPL [Member] | Mar. 31, 2013 Financial Transmission Rights [Member] WPL [Member]
Cumulative preferred stock rate | 5.10% | 8.375% | 4.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value, net derivative liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.4 | $ 0.3 | $ 1.1 |  |  |
Fair value, net derivative assets |  |  |  | $ 4.9 | $ 11.9 | $ (15.1) | $ (0.9) | $ 4.3 | $ 12.5 | $ (11.4) | $ 4.3 | $ 0.6 | $ (0.6) | $ (3.7) | $ (5.2) |  |  |  | $ 6.3 | $ 4.6 | $ 1.7

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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 Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 Reference 2: 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. + Details
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- Definition This element represents a liability 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 Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
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Balance Type: | credit
Period Type: | instant

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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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