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Benefit Plans (Summary Of Performance Unit Activity) (Details) (Performance Units [Member], USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |
Nonvested, January 1 (in shares/awards) | 63,665 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1] | 65,912 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1]
Granted (in shares/awards) | 17,837 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod | [1] | 20,422 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod | [1]
Vested (in shares/awards) | (22,845)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod | [1] | (20,751)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod | [1]
Forfeited (in shares/awards) | (93)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod | [1] | (311)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod | [1]
Nonvested, March 31 (in shares/awards) | 58,564 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1] | 65,272 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber | [1]
2012 Grant [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward] |  |  |  |
Vested (in shares/awards) | (22,845)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
Vested percentage of the target | 167.50%lnt_VestedPercentageOfTarget / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
Aggregate payout value | 1.6 us-gaap_EmployeeBenefitsAndShareBasedCompensation / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |
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 |
Aggregate payout value |  |  | 1.2 us-gaap_EmployeeBenefitsAndShareBasedCompensation / us-gaap_AwardDateAxis = lnt_TwoThousandElevenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |
[1] | Unit amounts represent the target number of performance units.

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- Definition Vested Percentage Of The Target + 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 equity-based payment instruments, excluding stock (or unit) options, that were forfeited 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + 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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