Commitments And Contingencies (Narrative) (Details) (USD $)	1 Months Ended	12 Months Ended		12 Months Ended																			12 Months Ended				12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended		12 Months Ended	1 Months Ended			12 Months Ended	1 Months Ended
	Jul. 31, 2012 subclass	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Workforce Subject to Collective Bargaining Arrangements [Member]	Dec. 31, 2012 Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]	Dec. 31, 2012 Defined Benefit Pension Plans [Member]		Dec. 31, 2011 Defined Benefit Pension Plans [Member]		Dec. 31, 2010 Defined Benefit Pension Plans [Member]		Dec. 31, 2012 Clean Air Interstate Rule [Member] MW	Dec. 31, 2012 Utility Maximum Achievable Control Technology [Member] Minimum [Member] MW	Dec. 31, 2012 Wisconsin State Mercury Rule [Member] lb_per_gw_hr	Dec. 31, 2012 Ozone NAAQS Rule [Member] ppm	Dec. 31, 2012 Fine Particle NAAQS Rule [Member]	Dec. 31, 2012 Nitrogen Dioxide NAAQS Rule [Member] ppb	Dec. 31, 2012 SO2 NAAQS Rule [Member] ppb	Dec. 31, 2012 EPA New Source Performance Standards for GHG [Member] lb MW	Dec. 31, 2012 Electric Generating Units Mercury Emission Reduction Percentage [Member] Wisconsin State Mercury Rule [Member]	Dec. 31, 2010 Electric Generating Units Mercury Emission Reduction Percentage [Member] Wisconsin State Mercury Rule [Member]	Dec. 31, 2012 Manufactured Gas Plant Sites [Member]	Dec. 31, 2012 Scenario, Forecast [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Performance guarantees outstanding, 2013 [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Performance guarantees outstanding, 2014 [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Performance guarantees outstanding, 2015 [Member]	Dec. 31, 2012 Alliant Energy and WPL [Member] Scenario, Forecast [Member] Capital Purchase Obligation [Member]	Dec. 31, 2012 IPL [Member] site	Dec. 31, 2011 IPL [Member]	Dec. 31, 2012 IPL [Member] Workforce Subject to Collective Bargaining Arrangements [Member]	Dec. 31, 2012 IPL [Member] Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]	Dec. 31, 2012 IPL [Member] Manufactured Gas Plant Sites [Member]	Dec. 31, 2012 IPL [Member] Scenario, Forecast [Member]	Dec. 31, 2012 WPL [Member] site	Dec. 31, 2011 WPL [Member]	Dec. 31, 2012 WPL [Member] Workforce Subject to Collective Bargaining Arrangements [Member]	Dec. 31, 2012 WPL [Member] Manufactured Gas Plant Sites [Member]	Dec. 31, 2012 WPL [Member] Scenario, Forecast [Member]	May 31, 2011 Cash Balance Plan [Member]	Sep. 30, 2012 Cash Balance Plan [Member] Defined Benefit Pension Plans [Member]	Aug. 31, 2012 Cash Balance Plan [Member] Defined Benefit Pension Plans [Member]	Dec. 31, 2011 Cash Balance Plan [Member] Defined Benefit Pension Plans [Member]	Feb. 28, 2013 Cash Balance Plan [Member] Defined Benefit Pension Plans [Member] Subsequent Event [Member]	Dec. 31, 2012 RMT [Member] Indemnification Agreement [Member]
Long-term Purchase Commitment [Line Items]
Minimum future commitments		$ 982,000,000																									$ 46,000,000	$ 537,000,000						$ 303,000,000
Interest crediting rate																																							8.20%
Aggregate additional payments to certain plan participants						100,000	[1]	10,200,000	[1]	0	[1]																															10,000,000
Number of new subclasses certified as a result of court opinion and order	2
Estimated damages from court opinion and order																																									18,700,000
Loss contingency, amount of plaintiff's attorney fees or costs																																								9,600,000
Loss contingency, amount of plaintiff's attorney fees or costs paid out of common fund																																								4,300,000
Loss contingency, damages awarded, value																																											6,400,000
Loss contingency, estimated amount																																												3,000,000
Performance guarantees outstanding		615,000,000																						270,000,000	294,000,000	51,000,000
Surety bonds																								18,000,000
Present value abandonment obligation		30,000,000
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 (in sites)																												3
Minimum remaining estimated cost incurred for investigation, remediation and monitoring																							18,000,000										16,000,000					2,000,000
Maximum remaining estimated cost incurred for investigation, remediation and monitoring																							42,000,000										38,000,000					4,000,000
Current and non-current environmental liabilities		$ 29,000,000	$ 33,600,000																			$ 29,000,000						$ 25,700,000	$ 28,400,000			$ 26,000,000		$ 3,300,000	$ 5,100,000		$ 3,000,000
Fossil fueled energy generating units capacity (in megawatts)												25	25						25
Energy generating units mercury emission reduction																				90.00%	40.00%
Annual concentration of mercury emissions (in pounds of mercury per gigawatt-hour)														0.008
Reduction of ozone primary standard level (in parts per million)															0.075
Annual standard fine particle NAAQS, current (in micrograms/cubic meters)																15
Annual standard fine particle NAAQS (in micrograms/cubic meters)																12
One hour NAAQS level (in parts per billion)																	100	75
Annual standard NAAQS (in parts per billion)																	53
Emissions rate limitation (in pounds of carbon dioxide per MWh)																			1,000
Concentration risk				55.00%	18.00%																									67.00%	44.00%					80.00%

[1] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan in 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in 2011 resulting in aggregate additional benefits of $10.2 million paid by Alliant Energy to certain former participants in the Cash Balance Plan in 2011. Alliant Energy recognized $10.2 million of additional benefits costs in 2011 related to these benefits. IPL recognized $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) and WPL recognized $3.4 million ($0.7 million directly assigned and $2.7 million allocated by Corporate Services) of additional benefits costs in 2011 related to these benefits. Refer to Note 13(c) for additional information regarding the Cash Balance Plan.