Benefit Plans (Fair Values Of Nonvested Performance Shares And Units) (Details) (USD $) In Millions, except Share data, unless otherwise specified																	3 Months Ended
	Mar. 31, 2012 Performance Shares [Member]		Dec. 31, 2011 Performance Shares [Member]		Mar. 31, 2011 Performance Shares [Member]		Dec. 31, 2010 Performance Shares [Member]		Mar. 31, 2012 Performance Units [Member]		Dec. 31, 2011 Performance Units [Member]		Mar. 31, 2011 Performance Units [Member]		Dec. 31, 2010 Performance Units [Member]		Mar. 31, 2012 2012 Grant [Member] Performance Shares [Member]	Mar. 31, 2012 2012 Grant [Member] Performance Units [Member]	Mar. 31, 2012 2011 Grant [Member] Performance Shares [Member]	Mar. 31, 2012 2011 Grant [Member] Performance Units [Member]	Mar. 31, 2012 2010 Grant [Member] Performance Shares [Member]	Mar. 31, 2012 2010 Grant [Member] Performance Units [Member]
Defined Benefit Plan Disclosure [Line Items]
Nonvested awards	170,611	[1]	236,979	[1]	240,897	[1]	234,518	[1]	66,804	[2]	42,996	[2]	46,534	[2]	23,128	[2]	45,612	24,686	62,170	21,693	62,829	20,425
Alliant Energy common stock closing price on March 31, 2012																	$ 43.32		$ 43.32		$ 43.32
Alliant Energy common stock average price on grant date																		43.05		38.75		32.56
Estimated payout percentage based on performance criteria																	93.00%	93.00%	105.00%	105.00%	161.00%	161.00%
Fair values of each nonvested award																	$ 40.29	$ 40.04	$ 45.49	$ 40.69	$ 69.75	$ 52.41

[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.