Commitments And Contingencies (Operating Expenses Purchase Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended																							3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011 IPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2011 Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Other [Member] IPL [Member]		Dec. 31, 2011 Other [Member] WPL [Member]		Dec. 31, 2011 Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 Natural Gas [Member] IPL [Member]	Dec. 31, 2011 Natural Gas [Member] WPL [Member]	Dec. 31, 2011 Coal [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Coal [Member] IPL [Member]		Dec. 31, 2011 Coal [Member] WPL [Member]		Dec. 31, 2011 SO2 Emission Allowances [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 SO2 Emission Allowances [Member] IPL [Member]		Dec. 31, 2007 SO2 Emission Allowances [Member] IPL [Member]	Sep. 30, 2011 SO2 Emission Allowances [Member] Alliant Energy And IPL [Member]	Dec. 31, 2011 Cross State Air Pollution Rule [Member] Alliant Energy And IPL [Member]	Dec. 31, 2011 Purchased Power [Member]	Dec. 31, 2011 Purchased Power [Member] IPL [Member]	Dec. 31, 2011 Purchased Power [Member] WPL [Member]		Dec. 31, 2011 Purchased Power [Member] DAEC (IPL) [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Purchased Power [Member] DAEC [Member] IPL [Member]		Dec. 31, 2011 Purchased Power [Member] Kewaunee [Member] WPL [Member]		Dec. 31, 2011 Purchased Power [Member] Kewaunee WPL [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Purchased Power [Member] Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Purchased Power [Member] Other [Member] IPL [Member]		Dec. 31, 2011 Purchased Power [Member] Other [Member] WPL [Member]		Dec. 31, 2011 2012 [Member]	Dec. 31, 2011 2012 [Member] IPL [Member]	Dec. 31, 2011 2012 [Member] WPL [Member]	Dec. 31, 2011 2012 [Member] Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2012 [Member] Other [Member] IPL [Member]		Dec. 31, 2011 2012 [Member] Other [Member] WPL [Member]		Dec. 31, 2011 2012 [Member] Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 2012 [Member] Natural Gas [Member] IPL [Member]	Dec. 31, 2011 2012 [Member] Natural Gas [Member] WPL [Member]	Dec. 31, 2011 2012 [Member] Coal [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2012 [Member] Coal [Member] IPL [Member]		Dec. 31, 2011 2012 [Member] Coal [Member] WPL [Member]		Dec. 31, 2011 2012 [Member] Coal [Member] Corporate Services [Member]	Dec. 31, 2011 2012 [Member] Purchased Power [Member]	Dec. 31, 2011 2012 [Member] Purchased Power [Member] IPL [Member]	Dec. 31, 2011 2012 [Member] Purchased Power [Member] WPL [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] DAEC (IPL) [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] DAEC [Member] IPL [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] Kewaunee [Member] WPL [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] Kewaunee WPL [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] Other [Member] IPL [Member]		Dec. 31, 2011 2012 [Member] Purchased Power [Member] Other [Member] WPL [Member]		Dec. 31, 2011 2013 [Member]	Dec. 31, 2011 2013 [Member] IPL [Member]	Dec. 31, 2011 2013 [Member] WPL [Member]	Dec. 31, 2011 2013 [Member] Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2013 [Member] Other [Member] IPL [Member]		Dec. 31, 2011 2013 [Member] Other [Member] WPL [Member]		Dec. 31, 2011 2013 [Member] Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 2013 [Member] Natural Gas [Member] IPL [Member]	Dec. 31, 2011 2013 [Member] Natural Gas [Member] WPL [Member]	Dec. 31, 2011 2013 [Member] Coal [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2013 [Member] Coal [Member] IPL [Member]		Dec. 31, 2011 2013 [Member] Coal [Member] WPL [Member]		Dec. 31, 2011 2013 [Member] Coal [Member] Corporate Services [Member]	Dec. 31, 2011 2013 [Member] Purchased Power [Member]	Dec. 31, 2011 2013 [Member] Purchased Power [Member] IPL [Member]	Dec. 31, 2011 2013 [Member] Purchased Power [Member] WPL [Member]		Dec. 31, 2011 2013 [Member] Purchased Power [Member] DAEC (IPL) [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2013 [Member] Purchased Power [Member] DAEC [Member] IPL [Member]		Dec. 31, 2011 2013 [Member] Purchased Power [Member] Kewaunee [Member] WPL [Member]		Dec. 31, 2011 2013 [Member] Purchased Power [Member] Kewaunee WPL [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2013 [Member] Purchased Power [Member] Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2013 [Member] Purchased Power [Member] Other [Member] WPL [Member]		Dec. 31, 2011 2014 [Member]	Dec. 31, 2011 2014 [Member] IPL [Member]	Dec. 31, 2011 2014 [Member] WPL [Member]	Dec. 31, 2011 2014 [Member] Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 2014 [Member] Natural Gas [Member] IPL [Member]	Dec. 31, 2011 2014 [Member] Natural Gas [Member] WPL [Member]	Dec. 31, 2011 2014 [Member] Coal [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2014 [Member] Coal [Member] IPL [Member]		Dec. 31, 2011 2014 [Member] Coal [Member] WPL [Member]		Dec. 31, 2011 2014 [Member] Purchased Power [Member]	Dec. 31, 2011 2014 [Member] Purchased Power [Member] IPL [Member]	Dec. 31, 2011 2014 [Member] Purchased Power [Member] DAEC (IPL) [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2014 [Member] Purchased Power [Member] DAEC [Member] IPL [Member]		Dec. 31, 2011 2015 [Member]	Dec. 31, 2011 2015 [Member] IPL [Member]	Dec. 31, 2011 2015 [Member] WPL [Member]	Dec. 31, 2011 2015 [Member] Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 2015 [Member] Natural Gas [Member] IPL [Member]	Dec. 31, 2011 2015 [Member] Natural Gas [Member] WPL [Member]	Dec. 31, 2011 2015 [Member] Coal [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2015 [Member] Coal [Member] IPL [Member]		Dec. 31, 2011 2015 [Member] Coal [Member] WPL [Member]		Dec. 31, 2011 2015 [Member] SO2 Emission Allowances [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2015 [Member] SO2 Emission Allowances [Member] IPL [Member]		Dec. 31, 2011 2016 [Member]	Dec. 31, 2011 2016 [Member] IPL [Member]	Dec. 31, 2011 2016 [Member] WPL [Member]	Dec. 31, 2011 2016 [Member] Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 2016 [Member] Natural Gas [Member] IPL [Member]	Dec. 31, 2011 2016 [Member] Natural Gas [Member] WPL [Member]	Dec. 31, 2011 2016 [Member] SO2 Emission Allowances [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 2016 [Member] SO2 Emission Allowances [Member] IPL [Member]		Dec. 31, 2011 Thereafter [Member]	Dec. 31, 2011 Thereafter [Member] IPL [Member]	Dec. 31, 2011 Thereafter [Member] WPL [Member]	Dec. 31, 2011 Thereafter [Member] Natural Gas [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 Thereafter [Member] Natural Gas [Member] IPL [Member]	Dec. 31, 2011 Thereafter [Member] Natural Gas [Member] WPL [Member]	Dec. 31, 2011 Thereafter [Member] SO2 Emission Allowances [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Thereafter [Member] SO2 Emission Allowances [Member] IPL [Member]		Dec. 31, 2011 Thereafter [Member] Purchased Power [Member]	Dec. 31, 2011 Thereafter [Member] Purchased Power [Member] IPL [Member]	Dec. 31, 2011 Thereafter [Member] Purchased Power [Member] Other [Member] Alliant Energy Corporation [Member]		Dec. 31, 2011 Thereafter [Member] Purchased Power [Member] Other [Member] IPL [Member]
Long-term Purchase Commitment, Amount	$ 1,374	$ 765	$ 427	$ 113	[1]	$ 65	[1]	$ 16	[1]	$ 306	$ 146	$ 160	$ 313	[2]	$ 98	[2]	$ 65	[2]	$ 34	[3]	$ 34	[3]	$ 34	$ 34	$ 34	$ 608	$ 422	$ 186	[4]	$ 412	[4],[5]	$ 412	[4],[5]	$ 149	[4]	$ 149	[4]	$ 47	[4]	$ 10	[4]	$ 37	[4]	$ 678	$ 362	$ 196	$ 89	[1]	$ 49	[1]	$ 8	[1]	$ 152	$ 83	$ 69	$ 149	[2]	$ 43	[2]	$ 18	[2]	$ 88	$ 288	$ 187	$ 101	[4]	$ 178	[4],[5]	$ 178	[4],[5]	$ 72	[4]	$ 72	[4]	$ 38	[4]	$ 9	[4]	$ 29	[4]	$ 475	$ 264	$ 149	$ 24	[1]	$ 16	[1]	$ 8	[1]	$ 60	$ 22	$ 38	$ 106	[2]	$ 26	[2]	$ 18	[2]	$ 62	$ 285	$ 200	$ 85	[4]	$ 200	[4],[5]	$ 200	[4],[5]	$ 77	[4]	$ 77	[4]	$ 8	[4]	$ 8	[4]	$ 108	$ 70	$ 38	$ 34	$ 14	$ 20	$ 40	[2]	$ 22	[2]	$ 18	[2]	$ 34	$ 34	$ 34	[4],[5]	$ 34	[4],[5]	$ 53	$ 29	$ 24	$ 23	$ 10	$ 13	$ 18	[2]	$ 7	[2]	$ 11	[2]	$ 12	[3]	$ 12	[3]	$ 35	$ 22	$ 13	$ 21	$ 8	$ 13	$ 14	[3]	$ 14	[3]	$ 25	$ 18	$ 7	$ 16	$ 9	$ 7	$ 8	[3]	$ 8	[3]	$ 1	$ 1	$ 1	[4]	$ 1	[4]
Individual commitments incurred	$ 1

[1] Includes individual commitments incurred during the normal course of business that exceeded $1 million at Dec. 31, 2011.

[2] IPL and WPL enter into coal transportation contracts that are directly assigned to their specific generating stations, the amounts of which are included in Alliant Energy's, IPL's and WPL's tables above. Also included in Alliant Energy's and IPL's tables is IPL's respective portion of coal and coal transportation contracts related to jointly-owned generating stations not operated by IPL. In addition, Corporate Services entered into system-wide coal contracts on behalf of IPL and WPL of $88 million for 2012 and $62 million for 2013 to allow flexibility for the changing needs of the quantity of coal consumed by each. Coal contract quantities are allocated to specific IPL or WPL generating stations at or before the time of delivery based on various factors including projected heat input requirements, combustion compatibility and efficiency. These system-wide coal contracts have not been directly assigned to IPL and WPL since the specific needs of each utility were not yet known as of Dec. 31, 2011 and therefore are excluded from IPL's and WPL's tables above.

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

[4] Includes payments required by PPAs for capacity rights and minimum quantities of MWhs required to be purchased. Refer to Note 21 for additional information on purchased power transactions.

[5] IPL is obligated to pay for capacity and energy delivered under the DAEC PPA. If energy delivered under the DAEC PPA is less than the targeted energy amount, an adjustment payment is made to IPL, which is reflected in IPL's energy adjustment clause.