XML 128 R95.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Benefit Plans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Benefit Plans [Line Items] |  |
Total plan assets, percentage of common stock (less than 1%) | 1.00% | 1.00%
Common stock percentage in assets held in 401(k) saving plans | 11.50% | 11.50%
Unrecognized compensation cost | $ 4.9 |
Alliant Energy common stock closing price on December 31, 2018 (in dollars per share) | $ 42.25 |
Percentage of base salary and performance-based compensation | 100.00% |
Carrying value of deferred compensation obligations | $ 21.0 | $ 21.8
Performance Shares and Performance Units [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Actual number of shares paid out upon vesting, minimum percentage of target shares | 0.00% |
Actual number of shares paid out upon vesting, maximum percentage of target shares | 200.00% |
Performance Restricted Stock Unit [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Actual number of shares paid out upon vesting, minimum percentage of target shares | 0.00% |
Actual number of shares paid out upon vesting, maximum percentage of target shares | 200.00% |
Restricted Stock Units [Member] |  |
Benefit Plans [Line Items] |  |
Performance period | 3 years |
Instrument valuation based on shares of common stock, number of shares | 1 |
Minimum [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 1 year |
Maximum [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 2 years |
IPL [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost | $ 2.7 |
WPL [Member] |  |
Benefit Plans [Line Items] |  |
Unrecognized compensation cost | 2.1 |
Other Postretirement Benefits Plans [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | 5.0 |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | 5.0 |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |
Benefit Plans [Line Items] |  |
Plan asset threshold for long-term allocation targets | $ 5.0 |
Omnibus Incentive Plan [Member] |  |
Benefit Plans [Line Items] |  |
Shares available for issuance under the Amended and Restated OIP | 6,800,000 |

X
- Definition Actual Number Of Shares Paid Out Upon Vesting Maximum + References No definition available. + Details
Name: | lnt_ActualNumberOfSharesPaidOutUponVestingMaximum
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Actual Number Of Shares Paid Out Upon Vesting Minimum + References No definition available. + Details
Name: | lnt_ActualNumberOfSharesPaidOutUponVestingMinimum
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Benefit Plans [Line Items] + References No definition available. + Details
Name: | lnt_BenefitPlansLineItems
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Carrying value of entity deferred compensation obligations interest and equity accounts + References No definition available. + Details
Name: | lnt_CarryingValueOfEntityDeferredCompensationObligationsInterestAndEquityAccounts
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Common stock percentage in assets held saving plans. + References No definition available. + Details
Name: | lnt_CommonStockPercentageInAssetsHeldSavingPlans
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Deferred Compensation Plan, Base Salary And Incentive Compensation, Percentage + References No definition available. + Details
Name: | lnt_DeferredCompensationPlanBaseSalaryAndIncentiveCompensationPercentage
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Defined Benefit Pension And Other Postretirement Benefits Plans, Total Plan Assets Percentage, On Common Stock + References No definition available. + Details
Name: | lnt_DefinedBenefitPensionAndOtherPostretirementBenefitsPlansTotalPlanAssetsPercentageOnCommonStock
Namespace Prefix: | lnt_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Defined Benefit Plan, Plan Asset Threshold for Long-term Allocation Targets + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanPlanAssetThresholdForLongTermAllocationTargets
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Instrument Valuation Based On Shares of Common Stock, Number of Shares + References No definition available. + Details
Name: | lnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsInstrumentValuationBasedOnSharesOfCommonStockNumberOfShares
Namespace Prefix: | lnt_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Period + References No definition available. + Details
Name: | lnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformancePeriod
Namespace Prefix: | lnt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Price of a single share of a number of saleable stocks of a company. + References No definition available. + Details
Name: | us-gaap_SharePrice
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_AwardTypeAxis=lnt_PerformanceSharesAndUnitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=lnt_PerformanceRestrictedStockUnitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
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: |

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_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PlanNameAxis=lnt_OmnibusIncentivePlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |