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Property, Plant and Equipment (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013 | Dec. 31, 2013
Property, Plant and Equipment [Line Items] |  |  |  |  |
Electric plant | $ 9,803.9 |  | $ 9,803.9 |  | $ 9,415.7
Allowance for funds used during construction | 8.4 | 7.0 | 17.5 | 12.6 |
George Neal Unit 3 [Member] | Alliant Energy and IPL [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Electric plant | 58.0 |  | 58.0 |  |
Allowance for funds used during construction |  |  | 4.0 |  |
Columbia Unit 2 [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Electric plant | 152.0 |  | 152.0 |  |
Allowance for funds used during construction |  |  | 8.0 |  |
Columbia Unit 1 [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |
Electric plant | 116.0 |  | 116.0 |  |
Allowance for funds used during construction |  |  | $ 7.0 |  |

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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 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: 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 3: 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 + Details
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