XML 25 R55.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2015 | Mar. 31, 2014
Production Tax Credits [Line Items] |  |
Production tax credits | $ 9.4 lnt_ProductionTaxCreditsNetOfStateTaxImpacts | $ 10.0 lnt_ProductionTaxCreditsNetOfStateTaxImpacts
WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Production tax credits | 5.1 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember | 5.4 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember
Cedar Ridge Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Nameplate Capacity (in megawatts) | 68 lnt_ElectricCapacityOfWindProject / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember |
Production tax credits | 1.2 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember | 1.2 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CedarRidgeWindProjectMember
Bent Tree - Phase I Wind Project [Member] | WPL [Member] |  |
Production Tax Credits [Line Items] |  |
Nameplate Capacity (in megawatts) | 201 lnt_ElectricCapacityOfWindProject / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_BentTreePhaseIWindProjectMember |
Production tax credits | 3.9 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_BentTreePhaseIWindProjectMember | 4.2 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_BentTreePhaseIWindProjectMember
Whispering Willow - East Wind Project [Member] | IPL [Member] |  |
Production Tax Credits [Line Items] |  |
Nameplate Capacity (in megawatts) | 200 lnt_ElectricCapacityOfWindProject / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_WhisperingWillowEastWindProjectMember |
Production tax credits | $ 4.3 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_WhisperingWillowEastWindProjectMember | $ 4.6 lnt_ProductionTaxCreditsNetOfStateTaxImpacts / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_WhisperingWillowEastWindProjectMember

X
- Definition Electric Capacity Of Wind Project + References No definition available. + Details
Name: | lnt_ElectricCapacityOfWindProject
Namespace Prefix: | lnt_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | lnt_ProductionTaxCreditsLineItems
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Production Tax Credits Net Of State Tax Impacts + References No definition available. + Details
Name: | lnt_ProductionTaxCreditsNetOfStateTaxImpacts
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=lnt_BentTreePhaseIWindProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=lnt_CedarRidgeWindProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=lnt_WhisperingWillowEastWindProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |