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Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $) | 3 Months Ended |  |  |
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Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Dec. 31, 2013
Performance Shares [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 144,215 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1] | 144,424 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1] | 145,926 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1] | 139,940 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1]
Performance Units [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 58,564 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2] | 63,665 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2] | 65,272 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2] | 65,912 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2]
2015 Grant [Member] | Performance Shares [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 45,403 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on March 31, 2015 (in dollars per share) | $ 63.00 lnt_CommonStockClosingPrice / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 100.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 63.00 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
2015 Grant [Member] | Performance Units [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 17,744 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 65.09 lnt_CommonStockClosingPriceGrantDate / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 100.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 65.09 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandFifteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
2014 Grant [Member] | Performance Shares [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 49,719 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on March 31, 2015 (in dollars per share) | $ 63.00 lnt_CommonStockClosingPrice / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 128.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 80.64 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
2014 Grant [Member] | Performance Units [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 19,440 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 53.77 lnt_CommonStockClosingPriceGrantDate / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 128.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 68.83 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
2013 Grant [Member] | Performance Shares [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 49,093 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on March 31, 2015 (in dollars per share) | $ 63.00 lnt_CommonStockClosingPrice / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 178.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 112.14 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
2013 Grant [Member] | Performance Units [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 21,380 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 47.58 lnt_CommonStockClosingPriceGrantDate / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 178.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 84.69 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
[1] | Share amounts represent the target number of performance shares.
[2] | Unit amounts represent the target number of performance units.

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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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