Commitments And Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended							6 Months Ended	3 Months Ended				6 Months Ended												6 Months Ended
	Dec. 31, 2010	Jun. 30, 2012	Jun. 30, 2012 IPL [Member] site	Jun. 30, 2012 WPL [Member] site	Jun. 30, 2012 RMT [Member] Vendor	Jun. 30, 2012 Manufactured Gas Plant Sites [Member] Alliant Energy [Member]	Jun. 30, 2012 Manufactured Gas Plant Sites [Member] IPL [Member]	Jun. 30, 2012 Manufactured Gas Plant Sites [Member] WPL [Member]	Jun. 30, 2011 Pension Benefits Costs [Member]	Jun. 30, 2012 Pension Benefits Costs [Member] Alliant Energy [Member]		Jun. 30, 2011 Pension Benefits Costs [Member] Alliant Energy [Member]		Jun. 30, 2012 Pension Benefits Costs [Member] Alliant Energy [Member]		Jun. 30, 2011 Pension Benefits Costs [Member] Alliant Energy [Member]		Jun. 30, 2012 Defined Benefit Pension [Member] Plaintiff [Member]	Jun. 30, 2012 Defined Benefit Pension [Member] Cash Balance Plan [Member]	Jun. 30, 2012 Minimum [Member]	Jun. 30, 2012 Maximum [Member]	Jun. 30, 2012 Performance guarantees outstanding, 2012 [Member]	Jun. 30, 2012 Performance guarantees outstanding, 2013 [Member]	Jun. 30, 2012 Performance guarantees outstanding, 2014 [Member]	Jun. 30, 2012 Performance guarantees outstanding, 2015 [Member]	Jun. 30, 2012 Scenario, Forecast [Member]	Jun. 30, 2012 Scenario, Forecast [Member] Alliant Energy [Member]	Jun. 30, 2012 Scenario, Forecast [Member] IPL [Member]	Jun. 30, 2012 Scenario, Forecast [Member] WPL [Member]	Jun. 30, 2012 Scenario, Forecast [Member] Capital Purchase Obligation [Member]	Jun. 30, 2012 Scenario, Forecast [Member] Capital Purchase Obligation [Member] IPL [Member]	Jun. 30, 2012 Scenario, Forecast [Member] Capital Purchase Obligation [Member] WPL [Member]	Jun. 30, 2012 EPA new source performance standards for GHG [Member] lb MW
Long-term Purchase Commitment [Line Items]
Minimum future commitments																											$ 1,062	$ 617	$ 341	$ 40	$ 8	$ 32
Interest crediting rate	8.20%
Estimated damages from court opinion and order																		18.7	17.9
Aggregate additional payments to certain plan participants									10.0	0	[1]	8.9	[1]	0	[1]	8.9	[1]
Number of vendors who filed lawsuits (in vendors)					3
Loss contingency accrual, carrying value, payments					17
Performance guarantees outstanding		595																				34	333	181	47
Surety bonds		88																				86	2
Present value abandonment obligation (in sites)		29
Number of sites with ownership interest (in sites)			40	14
Number of sites with no further action (in sites)			13	9
Sites with no further action for soil remediation			3
Minimum remaining estimated cost incurred for investigation, remediation and monitoring																										20		18	2
Maximum remaining estimated cost incurred for investigation, remediation and monitoring																										44		40	4
Current and non-current environmental liabilities						$ 31	$ 28	$ 3
Annual standard fine particle NAAQS, current (in micgrams/cubic meters)		15
Annual standard fine particle NAAQS (in micgrams/cubic meters)																				12	13
Fossil fueled energy generating units capacity (in megawatts)																																	25
Emissions rate limitation (in pounds of carbon dioxide per Mwh)																																	1,000

[1] Alliant Energy reached an agreement with the Internal Revenue Service (IRS), which resulted in a favorable determination letter for the Cash Balance Plan during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in the second quarter of 2011 resulting in aggregate additional benefits of $8.9 million paid to certain former participants in the Cash Balance Plan in August 2011. Alliant Energy recognized $8.9 million of additional benefits costs in the second quarter of 2011 related to these benefits. IPL was allocated $5.5 million ($2.2 million directly assigned and $3.3 million allocated by Corporate Services) and WPL was allocated $3.0 million ($0.6 million directly assigned and $2.4 million allocated by Corporate Services) of additional benefits costs in the second quarter of 2011 related to these benefits. Refer to Note 11(c) for additional information regarding the Cash Balance Plan.