Benefit Plans (Summary Of Performance Unit Activity) (Details) (Performance Units [Member])	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
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)	42,996	[1]	23,128	[1]
Granted (in shares)	24,686	[1]	23,975	[1]
Forfeited (in shares)	(878)	[1]	(4,107)	[1]
Nonvested, September 30 (in shares/awards)	66,804	[1]	42,996	[1]
Performance multiplier range, lower	0.00%
Performance multiplier range, upper	200.00%

[1] 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.