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v2.4.1.9

Income Taxes (Schedule Of Components Of Income Tax) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Current tax expense (benefit): |  |  |
Federal | $ 36.6 us-gaap_CurrentFederalTaxExpenseBenefit | $ 4.4 us-gaap_CurrentFederalTaxExpenseBenefit | $ (29.3)us-gaap_CurrentFederalTaxExpenseBenefit
State | 9.3 us-gaap_CurrentStateAndLocalTaxExpenseBenefit | (3.6)us-gaap_CurrentStateAndLocalTaxExpenseBenefit | 11.6 us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Deferred tax expense (benefit): |  |  |
Federal | 83.5 us-gaap_DeferredFederalIncomeTaxExpenseBenefit | 123.9 us-gaap_DeferredFederalIncomeTaxExpenseBenefit | 157.8 us-gaap_DeferredFederalIncomeTaxExpenseBenefit
State | 4.6 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit | 15.6 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit | 23.9 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Production tax credits | (31.3)lnt_ProductionTaxCreditsNetOfStateTaxImpacts | (31.0)lnt_ProductionTaxCreditsNetOfStateTaxImpacts | (24.8)lnt_ProductionTaxCreditsNetOfStateTaxImpacts
Investment tax credits | (1.6)us-gaap_InvestmentTaxCredit | (1.6)us-gaap_InvestmentTaxCredit | (1.7)us-gaap_InvestmentTaxCredit
Provision recorded as a change in uncertain tax positions, current | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent | 8.0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent
Provision recorded as a change in uncertain tax positions, deferred | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred | (0.4)lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred | (7.6)lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred
Provision recorded as a change in accrued interest | (0.1)lnt_ProvisionRecordedAsChangeInAccruedInterest | (0.5)lnt_ProvisionRecordedAsChangeInAccruedInterest | (0.2)lnt_ProvisionRecordedAsChangeInAccruedInterest
Income tax expense (benefit) | 44.3 us-gaap_IncomeTaxExpenseBenefit | 53.9 us-gaap_IncomeTaxExpenseBenefit | 89.4 us-gaap_IncomeTaxExpenseBenefit
IPL's tax benefit riders [Member] |  |  |
Current tax expense (benefit): |  |  |
IPL's tax benefit riders | (56.7)us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember | (52.9)us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember | (48.3)us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember
IPL [Member] |  |  |
Current tax expense (benefit): |  |  |
Federal | 11.3 us-gaap_CurrentFederalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | 11.7 us-gaap_CurrentFederalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (7.7)us-gaap_CurrentFederalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember
State | 3.4 us-gaap_CurrentStateAndLocalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (0.1)us-gaap_CurrentStateAndLocalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | 9.1 us-gaap_CurrentStateAndLocalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember
Deferred tax expense (benefit): |  |  |
Federal | 11.1 us-gaap_DeferredFederalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | 20.0 us-gaap_DeferredFederalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | 37.4 us-gaap_DeferredFederalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember
State | (6.2)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (0.8)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | 3.2 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember
Production tax credits | (14.0)lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember | (14.4)lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember | (12.5)lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember
Investment tax credits | (0.6)us-gaap_InvestmentTaxCredit / dei_LegalEntityAxis = lnt_IplMember | (0.6)us-gaap_InvestmentTaxCredit / dei_LegalEntityAxis = lnt_IplMember | (0.6)us-gaap_InvestmentTaxCredit / dei_LegalEntityAxis = lnt_IplMember
Provision recorded as a change in uncertain tax positions, current | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent / dei_LegalEntityAxis = lnt_IplMember | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent / dei_LegalEntityAxis = lnt_IplMember | 8.1 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent / dei_LegalEntityAxis = lnt_IplMember
Provision recorded as a change in uncertain tax positions, deferred | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred / dei_LegalEntityAxis = lnt_IplMember | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred / dei_LegalEntityAxis = lnt_IplMember | (8.2)lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred / dei_LegalEntityAxis = lnt_IplMember
Provision recorded as a change in accrued interest | 0 lnt_ProvisionRecordedAsChangeInAccruedInterest / dei_LegalEntityAxis = lnt_IplMember | (0.8)lnt_ProvisionRecordedAsChangeInAccruedInterest / dei_LegalEntityAxis = lnt_IplMember | (0.3)lnt_ProvisionRecordedAsChangeInAccruedInterest / dei_LegalEntityAxis = lnt_IplMember
Income tax expense (benefit) | (51.7)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (37.9)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (19.8)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | IPL's tax benefit riders [Member] |  |  |
Current tax expense (benefit): |  |  |
IPL's tax benefit riders | (56.7)us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember | (52.9)us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember | (48.3)us-gaap_CurrentIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember / us-gaap_RegulatoryLiabilityAxis = lnt_IplSTaxBenefitRiderMember
WPL [Member] |  |  |
Current tax expense (benefit): |  |  |
Federal | 0.6 us-gaap_CurrentFederalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | (5.7)us-gaap_CurrentFederalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 7.2 us-gaap_CurrentFederalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember
State | 4.4 us-gaap_CurrentStateAndLocalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 6.0 us-gaap_CurrentStateAndLocalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | (0.9)us-gaap_CurrentStateAndLocalTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember
Deferred tax expense (benefit): |  |  |
Federal | 88.9 us-gaap_DeferredFederalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 92.7 us-gaap_DeferredFederalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 81.1 us-gaap_DeferredFederalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember
State | 10.1 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 11.8 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 20.3 us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember
Production tax credits | (17.3)lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember | (16.6)lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember | (12.3)lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember
Investment tax credits | (1.0)us-gaap_InvestmentTaxCredit / dei_LegalEntityAxis = lnt_WplMember | (1.0)us-gaap_InvestmentTaxCredit / dei_LegalEntityAxis = lnt_WplMember | (1.1)us-gaap_InvestmentTaxCredit / dei_LegalEntityAxis = lnt_WplMember
Provision recorded as a change in uncertain tax positions, current | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent / dei_LegalEntityAxis = lnt_WplMember | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent / dei_LegalEntityAxis = lnt_WplMember | (0.1)lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsCurrent / dei_LegalEntityAxis = lnt_WplMember
Provision recorded as a change in uncertain tax positions, deferred | 0 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred / dei_LegalEntityAxis = lnt_WplMember | (0.4)lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred / dei_LegalEntityAxis = lnt_WplMember | 0.6 lnt_ProvisionRecordedAsChangeInUncertainTaxPositionsDeferred / dei_LegalEntityAxis = lnt_WplMember
Provision recorded as a change in accrued interest | (0.1)lnt_ProvisionRecordedAsChangeInAccruedInterest / dei_LegalEntityAxis = lnt_WplMember | 0.4 lnt_ProvisionRecordedAsChangeInAccruedInterest / dei_LegalEntityAxis = lnt_WplMember | (0.2)lnt_ProvisionRecordedAsChangeInAccruedInterest / dei_LegalEntityAxis = lnt_WplMember
Income tax expense (benefit) | $ 85.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | $ 87.2 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | $ 94.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember

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- Definition Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Current Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6509736 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Paragraph Question 1-7 + Details
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- Definition Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Current Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6509736 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 + Details
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- Definition Amount of deferred federal income tax expense (benefit) pertaining to income (loss) from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6510177 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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