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Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [Member], USD $)
In Millions, except Share data, unless otherwise specified
 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Performance Shares [Member]
    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]    
Nonvested, January 1 (in shares/awards) 236,979 [1] 234,518 [1]
Granted (in shares) 45,612 [1] 64,217 [1]
Vested (in shares) (111,980) [1],[2] (57,838) [1],[2]
Forfeited (in shares) (25,334) [1] (3,918) [1]
Nonvested, September 30 (in shares/awards) 145,277 [1] 236,979 [1]
Actual number of shares paid out upon vesting, minimum percentage of target shares 0.00%  
Actual number of shares paid out upon vesting, maximum percentage of target shares 200.00%  
Stock payout value $ 8.0 $ 1.6
Vested percentage of the target 162.50% 75.00%
Common stock shares from vested performance shares (in shares) 6,399 1,387
[1] Share amounts represent the target number of performance shares. Each performance share’s value is based on the 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] In the first quarter of 2012, 111,980 performance shares granted in 2009 vested at 162.5% of the target, resulting in payouts valued at $8.0 million, which consisted of a combination of cash and common stock (6,399 shares). In the first quarter of 2011, 57,838 performance shares granted in 2008 vested at 75% of the target, resulting in payouts valued at $1.6 million, which consisted of a combination of cash and common stock (1,387 shares).