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Income Taxes (Summary Of Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Operating Loss Carryforwards [Line Items] |  |
Deferred tax assets, net operating losses | $ 580 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards |
IPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred tax assets, net operating losses | 245 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / dei_LegalEntityAxis = lnt_IplMember |
WPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred tax assets, net operating losses | 213 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / dei_LegalEntityAxis = lnt_WplMember |
Federal [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Tax carryforwards, net operating losses | 970 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |
Deferred tax assets, net operating losses | 333 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |
Tax carryforwards, tax credits | 204 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |
Deferred tax assets, tax credits | 201 us-gaap_TaxCreditCarryforwardDeferredTaxAsset / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |
Federal [Member] | IPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Tax carryforwards, net operating losses | 468 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember |
Deferred tax assets, net operating losses | 161 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember |
Tax carryforwards, tax credits | 69 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember |
Deferred tax assets, tax credits | 68 us-gaap_TaxCreditCarryforwardDeferredTaxAsset / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_IplMember |
Federal [Member] | WPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Tax carryforwards, net operating losses | 376 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember |
Deferred tax assets, net operating losses | 129 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember |
Tax carryforwards, tax credits | 77 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember |
Deferred tax assets, tax credits | 75 us-gaap_TaxCreditCarryforwardDeferredTaxAsset / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember / dei_LegalEntityAxis = lnt_WplMember |
State [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Tax carryforwards, net operating losses | 881 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember | [1]
Deferred tax assets, net operating losses | 46 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember | [1]
State [Member] | IPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Tax carryforwards, net operating losses | 291 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_IplMember | [2]
Deferred tax assets, net operating losses | 16 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_IplMember | [2]
State [Member] | WPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Tax carryforwards, net operating losses | 171 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_WplMember | [3]
Deferred tax assets, net operating losses | $ 9 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember / dei_LegalEntityAxis = lnt_WplMember | [3]
After 2024 [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Net operating loss carryforwards, percentage expiring | 98.00%lnt_OperatingLossCarryforwardsNetPercentageExpiring / lnt_OperatingLossCarryforwardsExpirationDateAxis = lnt_After2024Member |
After 2024 [Member] | IPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Net operating loss carryforwards, percentage expiring | 96.00%lnt_OperatingLossCarryforwardsNetPercentageExpiring / dei_LegalEntityAxis = lnt_IplMember / lnt_OperatingLossCarryforwardsExpirationDateAxis = lnt_After2024Member |
After 2024 [Member] | WPL [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Net operating loss carryforwards, percentage expiring | 99.00%lnt_OperatingLossCarryforwardsNetPercentageExpiring / dei_LegalEntityAxis = lnt_WplMember / lnt_OperatingLossCarryforwardsExpirationDateAxis = lnt_After2024Member |
[1] | At December 31, 2014, Alliant Energy’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2033 with 98% expiring after 2024.
[2] | At December 31, 2014, IPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2031 with 96% expiring after 2024.
[3] | At December 31, 2014, WPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2031 with 99% expiring after 2024.

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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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