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Property, Plant and Equipment (Property, Plant and Equipment) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant anticipated to be retired early | 157.6 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPresentValueOfRegulatedAssetForPlantAbandonment | [1] | $ 157.8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPresentValueOfRegulatedAssetForPlantAbandonment | [1]
Total electric plant | 10,365.1 lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric |  | 9,415.7 lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric |
Gas plant in service | 946.2 lnt_PublicUtilitiesPropertyPlantAndEquipmentGas |  | 909.9 lnt_PublicUtilitiesPropertyPlantAndEquipmentGas |
Other plant in service | 539.3 lnt_PublicUtilitiesPropertyPlantAndEquipmentGeneral |  | 547.9 lnt_PublicUtilitiesPropertyPlantAndEquipmentGeneral |
Accumulated depreciation | (3,923.1)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation |  | (3,726.2)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation |
Net plant | 7,927.5 lnt_NetUtilityPlant |  | 7,147.3 lnt_NetUtilityPlant |
Construction work in progress | 479.1 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress |  | 677.9 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress |
Other, net | 22.3 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment |  | 22.3 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment |
Total utility | 8,428.9 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet |  | 7,847.5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet |
Non-regulated Generation, net | 240.1 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 | [2] | 249.4 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1 | [2]
Corporate Services and other, net | 269.4 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 | [3] | 229.6 lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine2 | [3]
Total non-regulated and other | 509.5 us-gaap_PropertyPlantAndEquipmentOther |  | 479.0 us-gaap_PropertyPlantAndEquipmentOther |
Total property, plant and equipment | 8,938.4 us-gaap_PropertyPlantAndEquipmentNet |  | 8,326.5 us-gaap_PropertyPlantAndEquipmentNet |
Non-regulated Generation, accumulated depreciation | 49.5 lnt_AccumulatedDepreciationOnPropertyPlantAndEquipmentNonRegulatedBusinessLine1 |  | 40.0 lnt_AccumulatedDepreciationOnPropertyPlantAndEquipmentNonRegulatedBusinessLine1 |
Corporate Services and other, accumulated depreciation | 229.1 lnt_AccumulatedDepreciationOnPropertyPlantAndEquipmentNonRegulatedBusinessLine2 |  | 214.2 lnt_AccumulatedDepreciationOnPropertyPlantAndEquipmentNonRegulatedBusinessLine2 |
Generation [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 5,463.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = lnt_ElectricGenerationMember | [4] | 4,792.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = lnt_ElectricGenerationMember | [4]
Electric - distribution [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 4,435.4 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = us-gaap_ElectricDistributionMember |  | 4,179.6 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = us-gaap_ElectricDistributionMember |
Other [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 309.1 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = us-gaap_OtherPlantInServiceMember |  | 286.3 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / us-gaap_PublicUtilityAxis = us-gaap_OtherPlantInServiceMember |
IPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant anticipated to be retired early | 0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPresentValueOfRegulatedAssetForPlantAbandonment / dei_LegalEntityAxis = lnt_IplMember | [1] | 0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPresentValueOfRegulatedAssetForPlantAbandonment / dei_LegalEntityAxis = lnt_IplMember | [1]
Total electric plant | 5,559.6 lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric / dei_LegalEntityAxis = lnt_IplMember |  | 5,034.9 lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric / dei_LegalEntityAxis = lnt_IplMember |
Gas plant in service | 474.0 lnt_PublicUtilitiesPropertyPlantAndEquipmentGas / dei_LegalEntityAxis = lnt_IplMember |  | 456.8 lnt_PublicUtilitiesPropertyPlantAndEquipmentGas / dei_LegalEntityAxis = lnt_IplMember |
Other plant in service | 298.8 lnt_PublicUtilitiesPropertyPlantAndEquipmentGeneral / dei_LegalEntityAxis = lnt_IplMember |  | 302.8 lnt_PublicUtilitiesPropertyPlantAndEquipmentGeneral / dei_LegalEntityAxis = lnt_IplMember |
Accumulated depreciation | (2,124.5)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation / dei_LegalEntityAxis = lnt_IplMember |  | (2,025.3)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation / dei_LegalEntityAxis = lnt_IplMember |
Net plant | 4,207.9 lnt_NetUtilityPlant / dei_LegalEntityAxis = lnt_IplMember |  | 3,769.2 lnt_NetUtilityPlant / dei_LegalEntityAxis = lnt_IplMember |
Construction work in progress | 325.0 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_IplMember |  | 346.4 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_IplMember |
Other, net | 21.8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember |  | 21.2 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember |
Total utility | 4,554.7 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_IplMember |  | 4,136.8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_IplMember |
Total property, plant and equipment | 4,554.7 us-gaap_PropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_IplMember |  | 4,136.8 us-gaap_PropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_IplMember |
IPL [Member] | Generation [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 2,872.4 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_ElectricGenerationMember | [4] | 2,513.2 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = lnt_ElectricGenerationMember | [4]
IPL [Member] | Electric - distribution [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 2,471.7 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricDistributionMember |  | 2,311.2 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricDistributionMember |
IPL [Member] | Other [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 215.5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = us-gaap_OtherPlantInServiceMember |  | 210.5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_IplMember / us-gaap_PublicUtilityAxis = us-gaap_OtherPlantInServiceMember |
WPL [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant anticipated to be retired early | 157.6 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPresentValueOfRegulatedAssetForPlantAbandonment / dei_LegalEntityAxis = lnt_WplMember | [1] | 157.8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPresentValueOfRegulatedAssetForPlantAbandonment / dei_LegalEntityAxis = lnt_WplMember | [1]
Total electric plant | 4,805.5 lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric / dei_LegalEntityAxis = lnt_WplMember |  | 4,380.8 lnt_PublicUtilitiesPropertyPlantAndEquipmentElectric / dei_LegalEntityAxis = lnt_WplMember |
Gas plant in service | 472.2 lnt_PublicUtilitiesPropertyPlantAndEquipmentGas / dei_LegalEntityAxis = lnt_WplMember |  | 453.1 lnt_PublicUtilitiesPropertyPlantAndEquipmentGas / dei_LegalEntityAxis = lnt_WplMember |
Other plant in service | 240.5 lnt_PublicUtilitiesPropertyPlantAndEquipmentGeneral / dei_LegalEntityAxis = lnt_WplMember |  | 245.1 lnt_PublicUtilitiesPropertyPlantAndEquipmentGeneral / dei_LegalEntityAxis = lnt_WplMember |
Accumulated depreciation | (1,798.6)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation / dei_LegalEntityAxis = lnt_WplMember |  | (1,700.9)us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAccumulatedDepreciation / dei_LegalEntityAxis = lnt_WplMember |
Net plant | 3,719.6 lnt_NetUtilityPlant / dei_LegalEntityAxis = lnt_WplMember |  | 3,378.1 lnt_NetUtilityPlant / dei_LegalEntityAxis = lnt_WplMember |
Leased Sheboygan Falls Energy Facility, net | 64.7 lnt_PublicUtilitiesPropertyPlantAndEquipmentLeased / dei_LegalEntityAxis = lnt_WplMember | [5] | 70.9 lnt_PublicUtilitiesPropertyPlantAndEquipmentLeased / dei_LegalEntityAxis = lnt_WplMember | [5]
Construction work in progress | 154.1 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_WplMember |  | 331.5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress / dei_LegalEntityAxis = lnt_WplMember |
Other, net | 0.5 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember |  | 1.1 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentOtherPropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember |
Total utility | 3,938.9 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_WplMember |  | 3,781.6 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_WplMember |
Total property, plant and equipment | 3,938.9 us-gaap_PropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_WplMember |  | 3,781.6 us-gaap_PropertyPlantAndEquipmentNet / dei_LegalEntityAxis = lnt_WplMember |
Leased Sheboygan Falls Energy Facility, accumulated depreciation | 59.1 lnt_AccumulatedAmortizationOnPublicUtilitiesPropertyPlantAndEquipmentLeased / dei_LegalEntityAxis = lnt_WplMember |  | 52.9 lnt_AccumulatedAmortizationOnPublicUtilitiesPropertyPlantAndEquipmentLeased / dei_LegalEntityAxis = lnt_WplMember |
WPL [Member] | Generation [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 2,590.6 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_ElectricGenerationMember | [4] | 2,278.8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = lnt_ElectricGenerationMember | [4]
WPL [Member] | Electric - distribution [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 1,963.7 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricDistributionMember |  | 1,868.4 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricDistributionMember |
WPL [Member] | Other [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Electric plant in service | 93.6 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = us-gaap_OtherPlantInServiceMember |  | $ 75.8 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentPlantInService / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilityAxis = us-gaap_OtherPlantInServiceMember |
WPL [Member] | Edgewater Unit 3 and Nelson Dewey Units 1 and 2 [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Property, plant and equipment, useful life | 10 years |  |  |
[1] | In 2013, WPL received approval from MISO to retire Edgewater Unit 3, and Nelson Dewey Units 1 and 2. WPL currently anticipates retiring these EGUs by December 31, 2015, contingent on completion of transmission network upgrades needed for system reliability. WPL is recovering the remaining net book value of these EGUs over a 10-year period beginning January 1, 2013 pursuant to a May 2012 PSCW order.
[2] | Less accumulated depreciation of $49.5 million and $40.0 million for Alliant Energy as of December 31, 2014 and 2013, respectively.
[3] | Less accumulated depreciation of $229.1 million and $214.2 million for Alliant Energy as of December 31, 2014 and 2013, respectively.
[4] | Includes various emission controls projects placed in service in 2014, which are discussed in “Emission Controls Projects” below.
[5] | Less accumulated amortization of $59.1 million and $52.9 million for WPL as of December 31, 2014 and 2013, respectively.

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