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Benefit Plans (Summary Of Performance Unit Activity) (Details) (Performance Units [Member], USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |  |  |
Nonvested, January 1 (in shares/awards) | 65,912 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1] | 64,969 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1] | 42,996 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1]
Granted (in shares/awards) | 20,422 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod | [1] | 22,201 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod | [1] | 24,686 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod | [1]
Vested (in shares/awards) | (20,751)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod | [1] | (19,760)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod | [1] | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod | [1]
Forfeited (in shares/awards) | (1,918)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod | [1] | (1,498)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod | [1] | (2,713)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod | [1]
Nonvested, December 31 (in shares/awards) | 63,665 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1] | 65,912 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1] | 64,969 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1]
Instrument valuation based on shares of common stock, number of shares | 1 lnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsInstrumentValuationBasedOnSharesOfCommonStockNumberOfShares |  |  |  |  |
Performance multiplier range, lower | 0.00%lnt_PerformanceUnitsAdjustmentForPerformanceMultiplierRangeMinimum |  |  |  |  |
Performance multiplier range, upper | 200.00%lnt_PerformanceUnitsAdjustmentForPerformanceMultiplierRangeMaximum |  |  |  |  |
2011 Grant [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |  |  |
Vested (in shares/awards) | (20,751)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardDateAxis = lnt_TwoThousandElevenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |
Vested percentage of the target | 147.50%lnt_VestedPercentageOfTarget / us-gaap_AwardDateAxis = lnt_TwoThousandElevenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |
Cash payout value | 1.2 us-gaap_EmployeeBenefitsAndShareBasedCompensation / us-gaap_AwardDateAxis = lnt_TwoThousandElevenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |
2010 Grant [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |  |  |
Vested (in shares/awards) |  |  | (19,760)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardDateAxis = lnt_TwoThousandTenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
Vested percentage of the target |  |  | 197.50%lnt_VestedPercentageOfTarget / us-gaap_AwardDateAxis = lnt_TwoThousandTenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
Cash payout value |  |  | 1.3 us-gaap_EmployeeBenefitsAndShareBasedCompensation / us-gaap_AwardDateAxis = lnt_TwoThousandTenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
[1] | Unit amounts represent the target number of performance units. Each performance unit’s value is based on the average price of one share of Alliant Energy’s common stock on the grant date of the award. The actual payout for performance units is dependent upon actual performance and may range from zero to 200% of the target number of units.

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- 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
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- Definition Costs and payments related to employee benefits and equity-based compensation, such as pension expense and contributions, other postretirement benefits expense and payments, stock or unit options expense, and amortization of restricted stock or unit. + 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=31042434&loc=d3e3602-108585 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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 (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + 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 (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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