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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2015
State Apportionment [Member] |  |
Income Tax [Line Items] |  |
Deferred income tax expense | $ 15 us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember |
Bonus Depreciation 2014 Return [Member] | Scenario, Forecast [Member] |  |
Income Tax [Line Items] |  |
Deductions in income tax return |  | 450 lnt_DeductionsInIncomeTaxReturn / us-gaap_IncomeTaxAuthorityAxis = lnt_BonusDepreciationTwoThousandFourteenReturnMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
IPL [Member] | State Apportionment [Member] |  |
Income Tax [Line Items] |  |
Deferred income tax expense | 8 us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_IplMember |
IPL [Member] | Bonus Depreciation 2014 Return [Member] | Scenario, Forecast [Member] |  |
Income Tax [Line Items] |  |
Deductions in income tax return |  | 245 lnt_DeductionsInIncomeTaxReturn / us-gaap_IncomeTaxAuthorityAxis = lnt_BonusDepreciationTwoThousandFourteenReturnMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
WPL [Member] | State Apportionment [Member] |  |
Income Tax [Line Items] |  |
Deferred income tax expense | 7 us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_IncomeTaxAuthorityAxis = lnt_StateApportionmentMember / dei_LegalEntityAxis = lnt_WplMember |
WPL [Member] | Bonus Depreciation 2014 Return [Member] | Scenario, Forecast [Member] |  |
Income Tax [Line Items] |  |
Deductions in income tax return |  | $ 190 lnt_DeductionsInIncomeTaxReturn / us-gaap_IncomeTaxAuthorityAxis = lnt_BonusDepreciationTwoThousandFourteenReturnMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember

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- Definition The difference between tax and book deductions that are expected to be reflected on income tax returns as applicable federal and state laws allow. + References No definition available. + Details
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- Definition Amount of deferred 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 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 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 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 6: 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 7: 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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