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Property, Plant and Equipment (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 1 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Apr. 30, 2015
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 6.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions | $ 9.1 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions |
IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 5.3 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember | 6.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember |
Edgewater Unit 5 [Member] | Alliant Energy and WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Construction work in progress | 110 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilityAxis = lnt_EdgewaterUnit5Member |  |
Allowance for funds used during construction | 4.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / us-gaap_PublicUtilityAxis = lnt_EdgewaterUnit5Member |  |
Marshalltown [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Fossil-fueled EGU capacity (in megawatts) | 650 lnt_FossilFueledEnergyGeneratingUnitsCapacity / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |
Marshalltown [Member] | Alliant Energy and IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Construction work in progress | 248 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |
Allowance for funds used during construction | 7.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / us-gaap_PublicUtilityAxis = lnt_MarshalltownGeneratingStationMember |  |
Customer Billing And Information System [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Capitalized expenditures | 71 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CustomerBillingAndInformationSystemMember |  |
Capitalized interest | 2 us-gaap_AccumulatedCapitalizedInterestCosts / us-gaap_PropertyPlantAndEquipmentByTypeAxis = lnt_CustomerBillingAndInformationSystemMember |  |
Subsequent Event [Member] | Gas distribution assets [Member] | IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Expected proceeds from asset sales |  |  | 11 us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasDistributionEquipmentMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Expected receipt of promissory note |  |  | $ 2 us-gaap_IncreaseDecreaseInOtherNoncurrentAssets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasDistributionEquipmentMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Fossil Fueled Energy Generating Units Capacity + References No definition available. + Details
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Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Carrying amount as of the balance sheet date of long-lived, depreciable Alliant Energy Corporate Services, Inc. and other plant. + References No definition available. + Details
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Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of accumulated interest costs capitalized as part of property, plant and equipment cost basis. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The increase (decrease) during the reporting period in other noncurrent operating assets not separately disclosed in the statement of cash flows. + References No definition available. + Details
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Balance Type: | credit
Period Type: | duration

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- Definition The cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
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- Details
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Period Type: | duration

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- Definition Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 + Details
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Balance Type: | credit
Period Type: | duration

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- Definition Period end amount of construction work in progress in public utility. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant

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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |