v2.4.0.6
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.