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Regulatory Matters (Electric Tax Benefit Rider) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Regulatory Liabilities [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | $ 10.8 us-gaap_IncreaseDecreaseInRegulatoryLiabilities | $ (90.8)us-gaap_IncreaseDecreaseInRegulatoryLiabilities | $ 16.4 us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Income tax expense (benefit) | 44.3 us-gaap_IncomeTaxExpenseBenefit | 53.9 us-gaap_IncomeTaxExpenseBenefit | 89.4 us-gaap_IncomeTaxExpenseBenefit
Alliant Energy and IPL [Member] | Electric [Member] | IPL's tax benefit riders [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Increase (decrease) in regulatory liabilities | (85.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (79.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (83.0)us-gaap_IncreaseDecreaseInRegulatoryLiabilities / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Income tax expense (benefit) | (50.0)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (46.0)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (48.0)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Alliant Energy and IPL [Member] | Electric [Member] | IPL's Tax Benefit Rider Decrease In Taxable Income [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Income tax expense (benefit) | $ (35.0)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderDecreaseInTaxableIncomeMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | $ (33.0)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderDecreaseInTaxableIncomeMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | $ (35.0)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderDecreaseInTaxableIncomeMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember

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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 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 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 Reference 4: 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=6907707&loc=d3e32639-109319 + 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=31042434&loc=d3e3602-108585 + Details
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