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Commitments And Contingencies (Schedule Of Environmental Liabilities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013
Current environmental liabilities | $ 2.0 us-gaap_AccruedEnvironmentalLossContingenciesCurrent | $ 3.6 us-gaap_AccruedEnvironmentalLossContingenciesCurrent
Non-current environmental liabilities | 13.5 us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent | 15.4 us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent
Total environmental liabilities | 15.5 us-gaap_AccrualForEnvironmentalLossContingencies | 19.0 us-gaap_AccrualForEnvironmentalLossContingencies
Minimum range of estimated future cost incurred for investigation, remediation and monitoring | 12 lnt_MinimumRemainingEstimatedCostIncurredForInvestigationRemediationAndMonitoring |
Maximum remaining estimated cost incurred for investigation, remediation and monitoring | 31 lnt_MaximumRemainingEstimatedCostIncurredForInvestigationRemediationAndMonitoring |
IPL [Member] |  |
Current environmental liabilities | 1.7 us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = lnt_IplMember | 2.8 us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = lnt_IplMember
Non-current environmental liabilities | 11.9 us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent / dei_LegalEntityAxis = lnt_IplMember | 13.6 us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent / dei_LegalEntityAxis = lnt_IplMember
Total environmental liabilities | 13.6 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = lnt_IplMember | 16.4 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = lnt_IplMember
Minimum range of estimated future cost incurred for investigation, remediation and monitoring | 11 lnt_MinimumRemainingEstimatedCostIncurredForInvestigationRemediationAndMonitoring / dei_LegalEntityAxis = lnt_IplMember |
Maximum remaining estimated cost incurred for investigation, remediation and monitoring | 29 lnt_MaximumRemainingEstimatedCostIncurredForInvestigationRemediationAndMonitoring / dei_LegalEntityAxis = lnt_IplMember |
WPL [Member] |  |
Current environmental liabilities | 0.3 us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = lnt_WplMember | 0.8 us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = lnt_WplMember
Non-current environmental liabilities | 1.6 us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent / dei_LegalEntityAxis = lnt_WplMember | 1.7 us-gaap_AccruedEnvironmentalLossContingenciesNoncurrent / dei_LegalEntityAxis = lnt_WplMember
Total environmental liabilities | 1.9 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = lnt_WplMember | 2.5 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = lnt_WplMember
Minimum range of estimated future cost incurred for investigation, remediation and monitoring | 1 lnt_MinimumRemainingEstimatedCostIncurredForInvestigationRemediationAndMonitoring / dei_LegalEntityAxis = lnt_WplMember |
Maximum remaining estimated cost incurred for investigation, remediation and monitoring | 2 lnt_MaximumRemainingEstimatedCostIncurredForInvestigationRemediationAndMonitoring / dei_LegalEntityAxis = lnt_WplMember |
Natural Gas Processing Plant [Member] |  |
Total environmental liabilities | 16.0 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = us-gaap_NaturalGasProcessingPlantMember |
Natural Gas Processing Plant [Member] | IPL [Member] |  |
Total environmental liabilities | 14.0 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = us-gaap_NaturalGasProcessingPlantMember |
Natural Gas Processing Plant [Member] | WPL [Member] |  |
Total environmental liabilities | $ 2.0 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis = us-gaap_NaturalGasProcessingPlantMember |

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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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