Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $) In Millions, except Share data, unless otherwise specified									9 Months Ended
	Sep. 30, 2011		Dec. 31, 2010		Sep. 30, 2010		Dec. 31, 2009		Sep. 30, 2011 Performance Shares [Member] 2011 Grant [Member]	Sep. 30, 2011 Performance Units [Member] 2011 Grant [Member]	Sep. 30, 2011 Performance Shares [Member] 2010 Grant [Member]	Sep. 30, 2011 Performance Units [Member] 2010 Grant [Member]	Sep. 30, 2011 Performance Shares [Member] 2009 Grant [Member]	Sep. 30, 2011 Performance Units [Member]		Dec. 31, 2010 Performance Units [Member]		Sep. 30, 2010 Performance Units [Member]
Nonvested awards	236,979	[1]	234,518	[1]	245,280	[1]	256,579	[1]	62,170	22,279	62,829	20,717	111,980	42,996	[2]	23,128	[2]	23,128	[2]
Alliant Energy common stock closing price on Sep. 30, 2011									$ 38.68		$ 38.68		$ 38.68
Alliant Energy common stock average price on grant date										38.75		32.56
Estimated payout percentage based on performance criteria									97.00%	97.00%	133.00%	133.00%	131.00%
Fair values of each nonvested award									$ 37.52	$ 37.59	$ 51.44	$ 43.30	$ 50.67

[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.