XML 94 R52.htm IDEA: XBRL DOCUMENT
v3.6.0.2

Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions |  | 12 Months Ended
--- | --- | ---
Apr. 20, 2016 | Dec. 31, 2017 | Dec. 31, 2016 USD ($) customer MW | Dec. 31, 2015 | Dec. 31, 2014
General: |  |  |  |  |
Common stock split conversion ratio (2:1) | 2 |  |  |  |
Adjustments for New Accounting Principle, Early Adoption [Member] |  |  |  |  |
New Accounting Standards: |  |  |  |  |
Decrease to deferred tax liabilities due to adoption of new accounting standard | $ |  |  | $ (3.1) |  |
Increase to retained earnings due to adoption of new accounting standard | $ |  |  | $ 3.1 |  |
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 |  |
Electric Generation Equipment [Member] | IPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  |  | 3.50% | 3.60% | 3.60%
Electric Generation Equipment [Member] | WPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  |  | 3.10% | 3.20% | 3.20%
Electric - distribution [Member] | IPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  |  | 2.40% | 2.40% | 2.50%
Electric - distribution [Member] | WPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  |  | 2.60% | 2.70% | 2.70%
Gas [Member] | IPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  |  | 3.30% | 3.20% | 3.30%
Gas [Member] | WPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  |  | 2.50% | 2.50% | 2.50%
Scenario, Forecast [Member] | Electric Generation Equipment [Member] | WPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  | 3.20% |  |  |
Scenario, Forecast [Member] | Electric - distribution [Member] | WPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  | 2.60% |  |  |
Scenario, Forecast [Member] | Gas [Member] | WPL [Member] |  |  |  |  |
Property, Plant and Equipment: |  |  |  |  |
Average rate of depreciation |  | 2.30% |  |  |

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 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 Amount of increase (decrease) in deferred obligations classified as other. + 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 (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInOtherDeferredLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Direct Effects of a Change in Accounting Principle -URI http://asc.fasb.org/extlink&oid=66092750 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22580-107794 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Indirect Effects of a Change in Accounting Principle -URI http://asc.fasb.org/extlink&oid=6515603 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accounting Change -URI http://asc.fasb.org/extlink&oid=6503790 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Retrospective Application -URI http://asc.fasb.org/extlink&oid=6523989 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22499-107794 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=64853466&loc=d3e22583-107794 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Change in Accounting Principle -URI http://asc.fasb.org/extlink&oid=6507316 + Details
Name: | us-gaap_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnEquityOrNetAssets1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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 Composite depreciation rate for public utility plants in service. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
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
- Definition Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB TOPIC 4.C) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Stock Split -URI http://asc.fasb.org/extlink&oid=6525746 + Details
Name: | us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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_PublicUtilityAxis=us-gaap_ElectricGenerationEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

X
- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |