XML 84 R52.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 USD ($) customer $ / unit MW | Dec. 31, 2014 USD ($)
Asset Impairments: |  |  |
Non-regulated Generation, net | $ | [1] | $ 229.3 | $ 240.1
IPL [Member] |  |  |
General: |  |  |
Generation and distribution of steam, number of customers served (in customers) | customer |  | 2 |
Property, Plant and Equipment: |  |  |
AFUDC accrual recorded, percentage of estimated CWIP |  | 100.00% |
Operating Revenues: |  |  |
Estimated recovery of overaccrued operating revenue, reflected on customer billing, maximum period |  | 2 years |
WPL [Member] |  |  |
Property, Plant and Equipment: |  |  |
AFUDC accrual recorded, percentage of estimated CWIP |  | 50.00% |
AFUDC rates, projects with approval |  | 100.00% |
Operating Revenues: |  |  |
Final settlement recovery of overaccrued operating revenue, reflected on customer billing, maximum period |  | 2 years |
Sheboygan Falls Energy Facility [Member] |  |  |
General: |  |  |
Fossil-fueled EGU capacity (in megawatts) | MW |  | 347 |
Capital lease, lease term |  | 20 years |
Franklin County Wind Farm [Member] |  |  |
General: |  |  |
Wind EGU capacity (in megawatts) | MW |  | 99 |
Asset Impairments: |  |  |
Non-regulated Generation, net | $ |  | $ 130.0 |
Customer Billing And Information System [Member] |  |  |
Property, Plant and Equipment: |  |  |
Property, plant and equipment, useful life |  | 12 years |
Software [Member] |  |  |
Property, Plant and Equipment: |  |  |
Property, plant and equipment, useful life |  | 5 years |
Minimum [Member] |  |  |
Asset Impairments: |  |  |
Average forward electricity price (in dollars per MWh) | $ / unit |  | 2 |
Maximum [Member] |  |  |
Asset Impairments: |  |  |
Average forward electricity price (in dollars per MWh) | $ / unit |  | 3 |
[1] | Less accumulated depreciation of $59.0 million and $49.5 million for Alliant Energy as of December 31, 2015 and 2014, respectively.

X
- Definition Capital Leases, Lease Term + References No definition available. + Details
Name: | lnt_CapitalLeasesLeaseTerm
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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
- Definition Estimated Recovery of Overaccrued Operating Revenue, Reflected on Customer Billing, Maximum Period + References No definition available. + Details
Name: | lnt_EstimatedRecoveryOfOveraccruedOperatingRevenueReflectedOnCustomerBillingMaximumPeriod
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Final Settlement Recovery of Overaccrued Operating Revenue, Reflected on Customer Billing, Maximum Period + References No definition available. + Details
Name: | lnt_FinalSettlementRecoveryOfOveraccruedOperatingRevenueReflectedOnCustomerBillingMaximumPeriod
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition General [Abstract] + References No definition available. + Details
Name: | lnt_GeneralAbstract
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Generation and Distribution of Steam, Number of Customers Served + References No definition available. + Details
Name: | lnt_GenerationAndDistributionOfSteamNumberOfCustomersServed
Namespace Prefix: | lnt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Carrying amount as of the balance sheet date of long-lived, depreciable Non-regulated Generation plant. + References No definition available. + Details
Name: | lnt_PropertyPlantAndEquipmentNonregulatedBusinessLine1
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Public Utilities, Allowance for Funds Used During Construction, Return Earned, Expected Percentage of Estimated Construction Work-in-process, Next Twelve Months + References No definition available. + Details
Name: | lnt_PublicUtilitiesAllowanceForFundsUsedDuringConstructionReturnEarnedExpectedPercentageOfEstimatedConstructionWorkInProcessNextTwelveMonths
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Recovery Rates For Projects With Approval + References No definition available. + Details
Name: | lnt_RecoveryRatesForProjectsWithApproval
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts. + References No definition available. + Details
Name: | us-gaap_DerivativeAverageForwardPrice
Namespace Prefix: | us-gaap_
Data Type: | us-types:perUnitItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_RevenuesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_TangibleAssetImpairmentChargesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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

X
- Details
Name: | us-gaap_PublicUtilityAxis=lnt_SheboyganFallsEnergyFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilityAxis=lnt_FranklinCountyWindProjectMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |