Benefit Plans (Summary Of Restricted Stock Activity) (Details) (USD $)	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 Time-Based Restricted Stock [Member]	Sep. 30, 2010 Time-Based Restricted Stock [Member]	Sep. 30, 2011 Performance-Contingent Restricted Stock [Member]	Sep. 30, 2010 Performance-Contingent Restricted Stock [Member]
Nonvested shares, Jan. 1	234,518	[1]	256,579	[1]			70,033	125,349	296,190	226,007
Granted during first quarter, Shares	64,217	[1]	72,487	[1]			5,000		64,217	72,487
Vested, Shares	(57,838)	[1],[2]			(57,838)		(38,633)	(51,736)	(53,274)
Forfeited, Shares	(3,918)	[1],[3]	(83,786)	[1],[3]		(57,100)	(600)	(1,300)	(5,395)	(2,304)
Nonvested shares, Sep. 30	236,979	[1]	245,280	[1]			35,800	72,313	301,738	296,190
Nonvested shares, Jan. 1, Weighted Average Fair Value							$ 32.27	$ 32.47	$ 32.32	$ 32.25
Granted during first quarter, Weighted Average Fair Value							$ 39.86		$ 38.75	$ 32.56
Vested, Weighted Average Fair Value							$ 34.60	$ 32.87	$ 37.93
Forfeited, Weighted Average Fair Value							$ 29.41	$ 32.78	$ 38.00	$ 32.56
Nonvested shares, Sep. 30, Weighted Average Fair Value							$ 30.87	$ 32.18	$ 32.60	$ 32.32

[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.