Commitments And Contingencies (Operating Expenses Purchase Obligations) (Details) (USD $) In Millions	9 Months Ended																																									12 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011 Alliant Energy Corporation [Member] Purchased Power [Member]		Sep. 30, 2011 Alliant Energy Corporation [Member] Purchased Power [Member] DAEC (IPL) [Member]		Sep. 30, 2011 Alliant Energy Corporation [Member] Purchased Power [Member] Kewaunee WPL [Member]		Sep. 30, 2011 Alliant Energy Corporation [Member] Purchased Power [Member] Other [Member]		Sep. 30, 2011 IPL [Member] Purchased Power [Member]		Sep. 30, 2011 IPL [Member] Purchased Power [Member] DAEC (IPL) [Member]		Sep. 30, 2011 IPL [Member] Purchased Power [Member] Other [Member]		Sep. 30, 2011 WPL [Member] Purchased Power [Member]		Sep. 30, 2011 WPL [Member] Purchased Power [Member] Kewaunee WPL [Member]		Sep. 30, 2011 WPL [Member] Purchased Power [Member] Other [Member]		Sep. 30, 2011 Alliant Energy Corporation [Member] Other [Member]		Sep. 30, 2011 IPL [Member] Other [Member]		Sep. 30, 2011 WPL [Member] Other [Member]		Sep. 30, 2011 Alliant Energy Corporation [Member] Natural Gas [Member]	Sep. 30, 2011 IPL [Member] Natural Gas [Member]	Sep. 30, 2011 WPL [Member] Natural Gas [Member]	Sep. 30, 2011 Alliant Energy Corporation [Member] Coal [Member]		Sep. 30, 2011 IPL [Member] Coal [Member]		Sep. 30, 2011 WPL [Member] Coal [Member]		Sep. 30, 2011 Corporate Services [Member] Coal Contracts [Member]	Sep. 30, 2011 Alliant Energy Corporation [Member] SO2 Emission Allowances [Member]		Sep. 30, 2011 IPL [Member] SO2 Emission Allowances [Member]		Dec. 31, 2007 IPL [Member] SO2 Emission Allowances [Member]	Sep. 30, 2011 SO2 Emission Allowances [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Cross State Air Pollution Rule [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Alliant Energy Corporation [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2011 WPL [Member]
Long-term Purchase Commitment, Amount		$ 657	[1]	$ 453	[1]	$ 165	[1]	$ 39	[1]	$ 465	[1]	$ 453	[1]	$ 12	[1]	$ 192	[1]	$ 165	[1]	$ 27	[1]	$ 90	[2]	$ 12	[2]	$ 20	[2]	$ 368	$ 175	$ 193	$ 335	[3]	$ 104	[3]	$ 66	[3]	$ 165	$ 34	[4]	$ 34	[4]	$ 34	$ 34	$ 34	$ 1,484	$ 790	$ 471
Individual commitments incurred	$ 1

[1] Includes payments required by purchased power agreements (PPAs) for capacity rights and minimum quantities of MWhs required to be purchased. Excludes contracts that are considered operating leases.

[2] Includes individual commitments incurred during the normal course of business that exceeded $1 million at Sep. 30, 2011.

[3] Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL that include minimum future commitments of $165 million that have not been directly assigned to IPL and WPL since the specific needs of each utility were not yet known as of Sep. 30, 2011.

[4] Refer to Note 1(b) for discussion of $34 million of charges recognized by Alliant Energy and IPL in the third quarter of 2011 for IPL's forward contracts to purchase SO2 emission allowances.