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Summary Of Significant Accounting Policies (Schedule Of Allowance For Funds Used During Construction) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | $ 21.9 | $ 12.0 | $ 18.0
IPL [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 8.4 | 5.8 | 5.5
WPL [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 13.5 | 6.2 | 12.5
Debt [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 7.8 | 4.4 | 6.8
Debt [Member] | IPL [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 3.2 | 2.3 | 2.5
Debt [Member] | WPL [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 4.6 | 2.1 | 4.3
Equity [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 14.1 | 7.6 | 11.2
Equity [Member] | IPL [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | 5.2 | 3.5 | 3.0
Equity [Member] | WPL [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Allowance for funds used during construction | $ 8.9 | $ 4.1 | $ 8.2

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- Definition Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 92 -Paragraph 8, 9, 12 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 90 -Paragraph 8 -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 15 -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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