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Regulatory Matters (Tax Benefit Riders) (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2015 | Sep. 30, 2014
Regulatory Liabilities [Line Items] |  |
Increase (decrease) in regulatory liabilities | $ 61.5 | $ (61.1)
Alliant Energy and IPL [Member] | Electric tax benefit rider [Member] |  |
Regulatory Liabilities [Line Items] |  |
Increase (decrease) in regulatory liabilities | (55.0) |
Alliant Energy and IPL [Member] | Gas tax benefit rider [Member] |  |
Regulatory Liabilities [Line Items] |  |
Increase (decrease) in regulatory liabilities | (9.0) |
Alliant Energy and IPL [Member] | Tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Increase (decrease) in regulatory liabilities | $ (64.0) |

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- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + 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 (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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