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Regulatory Matters (Gas Tax Benefit Rider) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Regulatory Liabilities [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | $ (83.8) | $ (63.0) | $ (67.8)
Income tax expense (benefit) | 66.7 | 59.4 | 70.4
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | (17.0) |  |
Alliant Energy and IPL [Member] | Gas [Member] | IPL's tax benefit riders [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | (6.0) | (12.0) | (12.0)
Income tax expense (benefit) | (3.0) | (7.0) | (7.0)
Alliant Energy and IPL [Member] | Gas [Member] | Decrease In taxable income from IPL's tax benefit rider [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Income tax expense (benefit) | $ (3.0) | $ (5.0) | $ (5.0)

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- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32639-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 + Details
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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=66902209&loc=d3e3602-108585 + Details
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