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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 6 Months Ended
Mar. 31, 2013
Jun. 30, 2013
Jun. 30, 2012
Performance Units [Member]
      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Nonvested, January 1 (in shares/awards) 64,969 [1] 64,969 [1] 42,996 [1]
Granted (in shares)   22,201 [1] 24,686 [1]
Vested (in shares) (19,760) (19,760) [1],[2] 0 [1],[2]
Forfeited (in shares)   (1,013) [1] (878) [1]
Nonvested, June 30 (in shares/awards)   66,397 [1] 66,804 [1]
Performance multiplier range, lower   0.00%  
Performance multiplier range, upper   200.00%  
Vested percentage of the target 197.50%    
Stock payout value $ 1.3    
[1] Unit amounts represent the target number of performance units. Each performance unit’s value is based on the closing 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.
[2] In the first quarter of 2013, 19,760 performance units granted in 2010 vested at 197.5% of the target, resulting in cash payouts valued at $1.3 million.