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Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [Member], USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
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] 256,579 [1]
Granted (in shares) 45,612 [1] 64,217 [1] 72,487 [1]
Vested (in shares) (111,980) [1],[2] (57,838) [1],[2] 0 [1],[2]
Forfeited (in shares) (25,334) [1],[3] (3,918) [1],[3] (94,548) [1],[3]
Nonvested, December 31 (in shares/awards) 145,277 [1] 236,979 [1] 234,518 [1]
Instrument valuation based on shares of common stock, number of shares 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  
2007 Grant [Member]
      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]      
Forfeited (in shares)     (57,100)
[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 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 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).
[3] In 2010, 57,100 performance shares granted in 2007 were forfeited without payout because the specified performance criteria for such shares were not met. The remaining forfeitures during 2012, 2011 and 2010 were primarily caused by retirements and voluntary terminations of participants.