Benefit Plans (Summary Of Performance Shares Activity) (Details) (USD $) In Millions, except Share data, unless otherwise specified	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 Shares [Member]
Nonvested shares, Jan. 1	234,518	[1]	256,579	[1]
Granted	64,217	[1]	72,487	[1]
Vested	(57,838)	[1],[2]			(57,838)
Forfeited	(3,918)	[1],[3]	(83,786)	[1],[3]		(57,100)
Nonvested shares, Sep. 30	236,979	[1]	245,280	[1]
Stock payout value					$ 1.6
Actual number of shares paid out upon vesting minimum							0.00%
Actual number of shares paid out upon vesting maximum							200.00%
Vested percentage of the target					75.00%
Common stock shares from vested performance shares					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 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).

[3] 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.