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Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015
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Carryforwards [Line Items] |
Net operating losses, deferred tax assets | $ 560 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
IPL [Member] |
Carryforwards [Line Items] |
Net operating losses, deferred tax assets | 230 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |
Carryforwards [Line Items] |
Net operating losses, deferred tax assets | 210 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / dei_LegalEntityAxis = lnt_WplMember
Federal [Member] |
Carryforwards [Line Items] |
Net operating losses, carryforward amount | 894 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember
Net operating losses, deferred tax assets | 306 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember
Tax credits, carryforward amount | 214 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember
Tax credits, deferred tax assets | 210 us-gaap_TaxCreditCarryforwardDeferredTaxAsset / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember
Federal [Member] | IPL [Member] |
Carryforwards [Line Items] |
Net operating losses, carryforward amount | 424 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember
Net operating losses, deferred tax assets | 145 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember
Tax credits, carryforward amount | 73 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember
Tax credits, deferred tax assets | 72 us-gaap_TaxCreditCarryforwardDeferredTaxAsset / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember
Federal [Member] | WPL [Member] |
Carryforwards [Line Items] |
Net operating losses, carryforward amount | 355 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember
Net operating losses, deferred tax assets | 122 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember
Tax credits, carryforward amount | 82 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember
Tax credits, deferred tax assets | 80 us-gaap_TaxCreditCarryforwardDeferredTaxAsset / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember
State [Member] |
Carryforwards [Line Items] |
Net operating losses, carryforward amount | 853 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember
Net operating losses, deferred tax assets | 44 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember
State [Member] | IPL [Member] |
Carryforwards [Line Items] |
Net operating losses, carryforward amount | 260 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_IplMember
Net operating losses, deferred tax assets | 13 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_IplMember
State [Member] | WPL [Member] |
Carryforwards [Line Items] |
Net operating losses, carryforward amount | 166 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_WplMember
Net operating losses, deferred tax assets | $ 8 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_WplMember

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 + Details
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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Carryforwards -URI http://asc.fasb.org/extlink&oid=6506874 + Details
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- Definition Amount of deferred tax asset, before valuation allowance, attributable to deductible other tax carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 30 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6907311&loc=d3e30536-109315 + Details
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