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Benefit Plans (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2009 | Jan. 31, 2014 | Dec. 31, 2013
Benefit Plans [Line Items] |  |  |  |
Total plan assets, percentage of common stock (less than 1%) | 1.00%lnt_DefinedBenefitPensionAndOtherPostretirementBenefitsPlansTotalPlanAssetsPercentageOnCommonStock |  |  |
Interest crediting rate, percentage above annual change in consumer price index |  | 3.00%lnt_MinimumRatingOfAmendmentInParticipantsInterestCredit |  |
Common stock percentage in assets held in 401(k) saving plans | 12.60%lnt_CommonStockPercentageInAssetsHeldSavingPlans |  |  | 11.30%lnt_CommonStockPercentageInAssetsHeldSavingPlans
Unrecognized compensation cost | $ 6.5 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |  |
Percentage of base salary and performance-based compensation | 100.00%lnt_DeferredCompensationPlanBaseSalaryAndIncentiveCompensationPercentage |  |  |
Performance Shares and Units [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance period | 3 years |  |  |
Expected volatility based on historical daily stock prices, historical period | 3 years |  |  |
Performance Shares [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance multiplier range, lower | 0.00%lnt_PerformanceUnitsAdjustmentForPerformanceMultiplierRangeMinimum / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |
Performance multiplier range, upper | 200.00%lnt_PerformanceUnitsAdjustmentForPerformanceMultiplierRangeMaximum / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |
Performance Units [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance multiplier range, lower | 0.00%lnt_PerformanceUnitsAdjustmentForPerformanceMultiplierRangeMinimum / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
Performance multiplier range, upper | 200.00%lnt_PerformanceUnitsAdjustmentForPerformanceMultiplierRangeMaximum / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
Minimum [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Unrecognized compensation cost recognized over a weighted average period | 1 year |  |  |
Minimum [Member] | Performance-Contingent Restricted Stock [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance period | 2 years |  |  |
Minimum [Member] | Performance Contingent Cash Awards [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance period | 2 years |  |  |
Maximum [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Unrecognized compensation cost recognized over a weighted average period | 2 years |  |  |
Maximum [Member] | Performance-Contingent Restricted Stock [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance period | 4 years |  |  |
Maximum [Member] | Performance Contingent Cash Awards [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Performance period | 4 years |  |  |
Other Postretirement Benefits Plans [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Plan asset threshold for long-term allocation targets | 5 lnt_DefinedBenefitPlanPlanAssetThresholdForLongTermAllocationTargets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  |  |
Other Postretirement Benefits Plans [Member] | Cash Balance Plan [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Final judgment from court order |  |  | 9.0 lnt_JudgmentOfDamagesByCourtMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_CashBalancePlanMember |
Plaintiffs' attorney's fees and costs |  |  |  | $ 6.7 us-gaap_LegalFees / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_CashBalancePlanMember
Omnibus Incentive Plan [Member] |  |  |  |
Benefit Plans [Line Items] |  |  |  |
Shares available for issuance under the OIP | 3.9 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_OmnibusIncentivePlanMember |  |  |

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- Details
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- Definition Common stock percentage in assets held saving plans. + References No definition available. + Details
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- Definition Deferred Compensation Plan, Base Salary And Incentive Compensation, Percentage + References No definition available. + Details
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- Definition Defined Benefit Pension And Other Postretirement Benefits Plans, Total Plan Assets Percentage, On Common Stock + References No definition available. + Details
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- Definition Defined Benefit Plan, Plan Asset Threshold for Long-term Allocation Targets + References No definition available. + Details
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- Definition Minimum rating of amendment in participants interest credit. + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Period + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Expected Volatility Based on Historical Daily Stock Prices, Historical Period + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The amount of expense provided in the period for legal costs incurred on or before the balance sheet date pertaining to resolved, pending or threatened litigation, including arbitration and mediation proceedings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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