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Benefit Plans (Estimated Amoritization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss) (Details) (Scenario, Forecast [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2015
Defined Benefit Pension Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | $ 35.4 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Prior service cost (credit) | (0.3)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | 35.1 us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Defined Benefit Pension Plans [Member] | IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 15.3 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Prior service cost (credit) | (0.1)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | 15.2 us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Defined Benefit Pension Plans [Member] | WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 16.8 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Prior service cost (credit) | 0.2 us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | 17.0 us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Other Postretirement Benefits Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 4.9 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Prior service cost (credit) | (11.3)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | (6.4)us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Other Postretirement Benefits Plans [Member] | IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 2.3 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Prior service cost (credit) | (6.1)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | (3.8)us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Other Postretirement Benefits Plans [Member] | WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Actuarial loss | 2.3 us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Prior service cost (credit) | (3.5)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Estimated amortization from regulatory assets, regulatory liabilities and AOCL | $ (1.2)us-gaap_DefinedBenefitPlanAmountToBeAmortizedFromAccumulatedOtherComprehensiveIncomeLossNextFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember

X
- Definition Amount included in accumulated other comprehensive income (AOCI) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position, including gain (loss), prior service cost (credit), and transition asset (obligation). + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount included in accumulated other comprehensive income (AOCI) related to prior service cost (credit) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration