Benefit Plans (Summary Of Performance Unit Activity) (Details)	9 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Mar. 31, 2011 Performance Shares [Member]	Mar. 31, 2010 Performance Shares [Member]	Sep. 30, 2011 Performance Units [Member]		Sep. 30, 2010 Performance Units [Member]
Nonvested shares, Jan. 1	234,518	[1]	256,579	[1]			23,128	[2]
Granted	64,217	[1]	72,487	[1]			23,975	[2]	23,795	[2]
Vested	(57,838)	[1],[3]			(57,838)
Forfeited	(3,918)	[1],[4]	(83,786)	[1],[4]		(57,100)	(4,107)	[2]	(667)	[2]
Nonvested shares, Sep. 30	236,979	[1]	245,280	[1]			42,996	[2]	23,128	[2]
Performance multiplier range, lower							0.00%
Performance multiplier range, upper							200.00%

[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.

[3] In the first quarter of 2011, 57,838 performance shares granted in 2008 vested at 75% of the target, resulting in a payout valued at $1.6 million, which consisted of a combination of cash and common stock (1,387 shares).

[4] In the first quarter of 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 the nine months ended Sep. 30, 2011 and 2010 were primarily caused by retirements and voluntary terminations of participants.