v2.4.0.6
Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $)
 9 Months Ended
Sep. 30, 2012
Performance Shares [Member]
Dec. 31, 2011
Performance Shares [Member]
Sep. 30, 2011
Performance Shares [Member]
Dec. 31, 2010
Performance Shares [Member]
Sep. 30, 2012
Performance Units [Member]
Dec. 31, 2011
Performance Units [Member]
Sep. 30, 2011
Performance Units [Member]
Dec. 31, 2010
Performance Units [Member]
Sep. 30, 2012
2012 Grant [Member]
Performance Shares [Member]
Sep. 30, 2012
2012 Grant [Member]
Performance Units [Member]
Sep. 30, 2012
2011 Grant [Member]
Performance Shares [Member]
Sep. 30, 2012
2011 Grant [Member]
Performance Units [Member]
Sep. 30, 2012
2010 Grant [Member]
Performance Shares [Member]
Sep. 30, 2012
2010 Grant [Member]
Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]                            
Nonvested awards (in shares) 145,277 [1] 236,979 [1] 236,979 [1] 234,518 [1] 66,804 [2] 42,996 [2] 42,996 [2] 23,128 [2] 45,612 24,686 45,235 21,693 54,430 20,425
Alliant Energy common stock closing price on September 30, 2012 (in dollars per share)                 $ 43.39   $ 43.39   $ 43.39  
Alliant Energy common stock average price on grant date (in dollars per share)                   $ 43.05   $ 38.75   $ 32.56
Estimated payout percentage based on performance criteria                 78.00% 78.00% 91.00% 91.00% 162.00% 162.00%
Fair values of each nonvested award (in dollars per share)                 $ 33.84 $ 33.58 $ 39.48 $ 35.26 $ 70.29 $ 52.74
[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] 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.