Benefit Plans (Summary Of Performance Shares Activity) (Details) (Performance Shares [Member], USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2011		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested shares, Jan. 1	234,518	[1]	234,518	[1]	256,579	[1]	208,579	[1]
Granted			64,217	[1]	72,487	[1]	152,735	[1]
Vested			(57,838)	[1],[2]			(84,633)	[1],[2]
Forfeited			(3,918)	[1],[3]	(94,548)	[1],[3]	(20,102)	[1],[3]
Nonvested shares, Dec. 31			236,979	[1]	234,518	[1]	256,579	[1]
Stock payout value			$ 1.6				$ 4.1
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				51,189

[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 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). In 2009, 84,633 performance shares granted in 2006 vested resulting in a payout valued at $4.1 million, which consisted of a combination of cash and common stock (51,189 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 2011, 2010 and 2009 were primarily caused by retirements and voluntary terminations of participants.