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Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $) | 12 Months Ended |  |  |
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Performance Shares [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 144,424 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1] | 139,940 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1] | 145,277 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1] | 236,979 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | [1]
Performance Units [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 63,665 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2] | 65,912 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2] | 64,969 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2] | 42,996 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember | [2]
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_TwoThousandFourteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on December 31, 2014 (in dollars per share) | $ 66.42 lnt_CommonStockClosingPrice / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 125.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 83.03 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenGrantMember / 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_TwoThousandFourteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 53.77 lnt_CommonStockAveragePriceGrantDate / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 125.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 67.21 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandFourteenGrantMember / 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 December 31, 2014 (in dollars per share) | $ 66.42 lnt_CommonStockClosingPrice / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 160.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 106.27 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_CommonStockAveragePriceGrantDate / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 160.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 76.13 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandThirteenGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
2012 Grant [Member] | Performance Shares [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 45,612 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on December 31, 2014 (in dollars per share) | $ 66.42 lnt_CommonStockClosingPrice / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 168.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 111.25 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |  |  |  |  |
2012 Grant [Member] | Performance Units [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Nonvested awards (in shares/awards) | 22,845 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Alliant Energy common stock closing price on grant date (in dollars per share) | $ 43.05 lnt_CommonStockAveragePriceGrantDate / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Estimated payout percentage based on performance criteria | 168.00%lnt_EstimatedPayoutPercentage / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
Fair values of each nonvested award (in dollars per share) | $ 72.11 lnt_EmployeeServiceShareBasedCompensationFairValuesOfEachNonvestedAwards / us-gaap_AwardDateAxis = lnt_TwoThousandTwelveGrantMember / us-gaap_AwardTypeAxis = lnt_PerformanceUnitsMember |  |  |  |  |  |  |
[1] | Share amounts represent the target number of performance shares. Each performance share’s value is based on the closing market price of one share of Alliant Energy’s common stock at the end of the performance period. The actual number of shares that will be paid out upon vesting is dependent upon actual performance and may range from zero to 200% of the target number of shares.
[2] | 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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